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Geology / Geology Group diary (37)
December 15, 2018, 04:52:40 pm
The geology Group met at Merlin's Bridge village hall at 10.30 am on Wednesday 12 December 2018.
The topic for this meeting wasBRITISH QUATERNARY GEOLOGY.
Since the end of the Tertiary Period around 2.5 Ma, the climate of Britain has fluctuated between cold glacial episodes and warmer temperate episodes. These climatic cycles are thought to be controlled by predictable changes in solar radiation (Milankovitch cycles). The Quaternary is divided into the Pleistocene epoch that extends up to 10,000 years ago, and the present day Holocene which is probably the latest interglacial phase. It is only during the last 300,000 years that we have direct evidence of glaciation in Britain where three major glacial episodes and interglacials can be recognised.
The earliest deposits of the Pleistocene are those of the Red Crag of East Anglia. These are shallow water marine sediments containing molluscs that indicate a cool temperate climate. They were laid down as sands and gravels on the margins of the North Sea basin. Today the iron stained sands of the Red Crag are well exposed in the cliffs of Walton on Naze. There are several more younger crag deposits overlying the Red Crag; these include the Norwich Crag and the Weybourne Crag. The latter was the last marine deposit in Britain and contains a high proportion of  molluscan arctic fauna; in fact there is a gradual decrease in the percentage of warm water shells throughout the Lower Pleistocene indicating a cooling of the climate. The succeeding Cromer Forest Bed  comprise two freshwater beds and  an estuarine bed that were laid down around 350,000 years ago. Large numbers of mammalian remains have been found in these beds including deer, sabre toothed tiger, hyaena, rhino and mammoth. Many bones appear to have been washed down by rivers along with plant remains that indicate a temperate climate.
However, towards the top of the sequence ice wedges and periglacial soils suggest the onset of the beginning of the Anglian glaciation (300-250,000 years ago) This is the earliest ice advance in Britain that  has left depositional evidence. The ice extended as far south as the Bristol Channel and the Thames valley and the course of the Thames was diverted by the advancing ice front southwards to its present position. In East Anglia the North Sea Drift (or Cromer Till) is formed of glacial till (also known as boulder clay) deposited as the ice sheets finally retreated. The till contains erratics from Scandinavia including a distinctive dark blue igneous rock called larvikite. However, the largest erratics are locally derived chalk rafts up to 100 metres in length; after subglacial plucking from the bed of what is now the North Sea, the blocks were moved by the ice sheet and then deposited when the ice melted. Deformation structures known as Contorted Drift can be seen in the coastal exposures where glacial sediments have been deformed by ice pressure and movement. The Cromer Ridge (14 kms long) is a conspicuous landform representing a terminal moraine 90 metres high that was formed along the margins of  successive ice sheets moving southwards towards the north Norfolk coast. The coast around Cromer displays sections through the moraine showing its internal structure.
The Hoxnian interglacial represents a return to temperate conditions after the Anglian glaciation. A rich flora and fauna include oak and alder plus mammalian remains but also skull fragments of early Homo sapiens. Sea level was 20-30 metres above the present day level as melting ice increased the volume of sea water.  The next glacial phase is known as the Wolstonian (200-130 years ago) when the ice sheets extended south across the Midlands. The vast proglacial Lake Harrison was formed when meltwater was ponded up between the ice front and the Cotswold escarpment. The succeeding Ipswichian interglacial was characterised by temperate conditions with an abundance of oak woodland as shown by. pollen analysis. Numerous mammals roamed the lowlands including elephant, bison, deer and rhinos and these were hunted by early humans. Eventually, the climate became cooler and the Devensian glaciation began around 115,000 years ago. The ice front did not reach as far south as in previous glaciations but ice lobes extended over the lowlands as in the Vale of York, the Cheshire plain and the Irish Sea. Terminal moraines like those at York and Escrick and other depositional features (drumlins, eskers and kames) are well preserved since they are less weathered than those of earlier glaciations. During recession of the ice front meltwater was again ponded up between the ice and high ground. The eastward drainage of the Vale of Pickering was blocked by glacial debris and the Derwent diverted southwards to the Ouse through the Kirkham Abbey overflow channel. A similar drainage diversion took place when the Severn which formerly flowed north, was blocked by ice and Lake Lapworth created. The waters later overflowed through the Ironbridge Gorge to create a southerly route for the river.

Some 10,000 years ago rising temperature caused the ice to melt and sea level rose by up to 100 metres. This was known as the Flandrian transgression when the English Channel was created along with numerous drowned estuaries (rias) such as the Milford haven, the Fal estuary and Poole harbour. The raised beaches in western Scotland for example, are evidence of higher sea levels at this time, but they also demonstrate the effect of isostatic uplift because when the ice melts, the land rises as the weight of ice is removed.
Landforms produced by Glaciation.
During the Pleistocene glacial phases highland Britain was covered by ice sheets at least 1000  metres thick. Ice moved outwards from the Scottish highlands, the Lake District and the Welsh mountains and valley glaciers moved down all the major valleys. The preglacial valleys were scoured and deepened by moving ice into U shaped troughs. Tributary streams may be left in hanging valleys above the main valley creating spectacular waterfalls. The glaciers were nourished in great hollows known as corries or cwms that were excavated on the mountain sides. The corries often became separated by knife edged ridges called arêtes. Bare rock outcrops on the valley floor were smoothed on the upflow side and plucked on the leeward side to form distinctive roche moutonées. Long ribbon lakes occupy some over deepened valleys, as for example in the Lake District.
Glaciated  lowland areas are often covered in drift materials such as sands, gravel and boulder clay ( glacial till). Boulders that have been transported by ice from distant sources are called erratics and these are useful in determining the direction of ice flow. The Austwick gritstone erratics in North Yorkshire weigh several tonnes and have been moved a few kilometres on to the limestone pavement. Meltwater flowing under a stagnant ice sheet often forms sinuous ridges of sand and gravel known as eskers. The Blakeney esker in Norfolk is a good example of this type of feature. Where boulder clay is deposited beneath an ice sheet in a valley it may be moulded into elliptical shaped mounds called drumlins that are aligned in the direction of ice flow. Moraines are accumulations of ill-sorted glacial debris that have been transported by ice and then deposited when the ice melts; terminal moraines form at the ice front marking the maximum advance of the ice; lateral moraines form along the side of a glacier.
John Downes
Geology / A Big Thank You
December 13, 2018, 03:02:50 pm
Dear Geology Group members,

This is a note to thank you all for your cards, good wishes and kind comments now that I am leaving Pembrokeshire. It has been a pleasure and a privilege to be able to talk about geology to such an enthusiastic group. Thank you for your support and interest over the years. I wish you all the very best for the future.

Kind Regards,

Geology / Geology Group Diary (36)
November 14, 2018, 09:34:30 pm
The Geology group met at 10.30am at Merlin's Bridge village hall on Wednesday 14 November 2018. The monthly topic was TERTIARY GEOLOGY IN BRITAIN.

The Tertiary period began around 65 Ma and continued up to 2 Ma.  During the break up of the super continent Pangaea, the North American plate and the Eurasian plate rifted apart to form the North Atlantic Ocean. In Palaeocene times (65-56 Ma) Britain was situated above a mantle plume (sometimes referred to as a hot spot) where the earth's crust was under stress from plate rifting. As a result fissures opened up along a line from Ireland to the Hebrides from which basaltic lavas were extruded and igneous centres developed, surrounded by ring dykes and cone sheets and radiating dyke swarms. A vast lava field (1.8 million km2) called the Thulean Plateau extended from West Scotland through the Faroes, to Iceland and Greenland.
The Plateau Basalts
Up to 2000 metres thickness of basalt lavas were erupted in Antrim, Mull and Skye during the Palaeocene. Even today after extensive erosion, the lavas are 1800 metres thick on Mull. The individual lava flows give rise to stepped topography which is caused by the weathering back of the softer vesicular slag at the top of each flow. Between the lava flows there are often found red lateritic soils. These iron rich deposits contain plant remains, particularly in the leaf beds of SW Mull where a temperate flora of ginkgo, plane, hazel and oak occur. The leaves are well preserved and are thought to have fallen into a shallow lake. In 1819 a magnificent specimen of a tree trunk was discovered embedded in lavas in the cliffs of western Mull; it is known as MacCulloch's tree!
The basalt lavas of the Giant's Causeway and the Isle of Staffa are world famous for their hexagonal columnar jointing. The columns result from the cooling and contraction of the lavas under perfectly stable conditions and they form perpendicular to the base and surface of the flow. The top of the lava flow would be filled with escaping gases which produced a vesicular slaggy texture. On the island of Eigg the lava cooled so quickly that it produced a glassy rock called pitchstone (similar to obsidian) that forms the Sgurr of Eigg.
The Igneous Complexes
There are several major plutonic centres extending northwards from the Mourne Mts in Northern Ireland through the Western Isles of Arran, Mull, Ardnamurchan, Rum and Skye. These centres represent the remains of Tertiary volcanoes and their underlying magma chambers.
On Skye the magma was emplaced in a sequence of Precambrian rocks (Lewisian gneiss and Torridonian sandstone overlain by basalt lavas. The magma was rich in mafic minerals (plagioclase, pyroxene and olivine). Layered gabbros were produced by the settling out of denser minerals and basaltic volcanoes erupted at the surface. Today we see the Cuillin Hills formed from the deeply eroded gabbronic magma chamber. However, the nearby Red Hills are formed of granitic magma produced within the same igneous complex. The granites were probably formed by the melting of the Precambrian basement rocks. In the Cuillins many minor intrusions occur as cone sheets rising out of the volcanic conduit and sloping outwards in a circle around the volcanic centre.
The island of Mull is almost entirely formed of an extinct volcano and its eroded magma chamber with three major eruptive centres within the complex. Numerous cone sheets form a distinctive arcuate pattern around central Mull. However, the Mull and Ardnamurchan volcanic centres are also characterised by the presence of ring dykes. These are cylindrical intrusions produced by magma being forced up through ring faults surrounding a central area of subsidence. As magma is removed from the magma chamber the centre of the complex sinks creating a caldera of subsidence. Think of a full wine bottle as a magma chamber and the cork representing the overlying rocks. Force the cork downwards and wine will be forced upwards around the cork creating a 'ring dyke'.
Dyke swarms occur throughout the Tertiary volcanic province and they all demonstrate a NW-SE trend. They are thought to be associated with shearing stresses produced by the opening of the North Atlantic during the early Palaeocene. Some 300 dykes can be seen on the south coast of Arran and these represent a7% crustal extension in a 20 km section. Perhaps the most famous Tertiary dyke is the Cleveland dyke in NE England. This can be traced for 400 kms from Mull to North Yorkshire and is thought to have been emplaced as a single pulse within a few days!
Tertiary sediments in Southern England
Whilst volcanoes were erupting in the north west of Britain, sedimentation was taking place in southern England. During the Palaeocene and Eocene epochs the London and Hampshire basins were subject to alternate marine transgression and regression.
London Basin. Initially the sea advanced from the east over the eroded chalk surface and laid down the Thanet Sands which are best exposed along the North Kent coast at Herne Bay.  Later, as the sea regressed the Reading Beds were deposited by meandering rivers flowing eastwards across mudflats . These continental type beds consist mainly of sands and clays but locally where silica cementation occurs, the sands form sarcen stones. These massive stones are often found on the chalk surface after the unconsolidated Reading beds have been removed by erosion. Sarcens can be seen on the Marlborough Downs on the western margin of the London Basin and they were used to build Stonehenge and other prehistoric monuments. Cemented pebble beds also occur such as the Hertfordshire Puddingstone; a superb example of a natural concrete! The succeeding cycle of sedimentation begins with a marine transgression that produced the London Clay up to 150 metres in thickness.  There are beds of calcareous concretions including septarian nodules at various levels within the London Clay. Numerous bivalves, brachiopods and gastropods provide evidence of the shallow marine environment. Sharks teeth are commonly found as fossils. .However, over 500 plant species have been recorded including mangroves, palms, laurel and magnolia which suggests a tropical climate existed in Eocene times. The plants and seeds were probably brought down by rivers and washed out to sea to be preserved in the silty marine sediments. A return to continental type conditions occurred as the sea shallowed and regressed and the Bagshot Sands were deposited in an estuarine environment. These beds are well developed in the western part of the London Basin giving rise to heathlands much used by the military around Aldershot and Bagshot.
In the Hampshire Basin (which includes the IOW) several cycles of sedimentation can be distinguished in the Hampshire Basin which roughly correspond to those in the London basin.  One of the best places to examine the Palaeogene succession is Alum Bay on the NW side of the IOW. Here the cliffs are formed mainly of multicoloured sands and clays (Bracklesham Beds) that are stained red, brown and green by iron compounds derived from pyrite (FeS2) which is oxidised above high tide level. There are also brown coloured beds of lignite and several plant beds providing evidence of a subtropical climate. The overlying Barton Beds are best examined on the Hampshire coast at Barton-on-Sea, east of Christchurch. Here the Barton Clay yields numerous bivalves (eg.Cardita) and gastropods (eg.Turritella; Volutospina) and sharks'teeth.
Much of the eastern part of East Anglia contains marine shelly sands known as Crag deposits. The Coralline Crag is of Pliocene age has an abundance of fossil bivalves, gastropods and brachiopods. The overlying Red Crag, Norwich Crag and Weybourne Crag form the lower part of the Pleistocene sequence. The molluscan fauna indicate a gradual climatic cooling from subtropical species in the Coralline Crag to boreal species in the Weybourne Crag, heralding the onset of the Pleistocene Ice Age.
The Alpine Orogeny.
The Alpine fold mountains were formed when Africa collided with Eurasia and the Tethys ocean crust was subducted northwards beneath the Eurasian plate. The Pyrenees, Alps, Apennines, Carpathians and Caucasus ranges were all formed by the Alpine orogeny, although some like the Pyrenees originated as Variscan structures. These earth movements occurred throughout the Tertiary but reached a peak during the Miocene when the Mediterranean was formed as a large saline basin on the site of the vanished Tethys Ocean. In southern Britain the ripple effect of Alpine crustal movements produced structures such as the Wealden anticline, the London and Hampshire synclinal basins and the Purbeck and IOW monocline.

Geology / Geology Group Diary (35)
October 11, 2018, 08:24:45 pm
The Geology Group met at 10.30am on Wednesday 10 October 2018 at Merlin's Bridge Village hall. 20 members present. The topic this month was...THE CRETACEOUS ROCKS OF BRITAIN.
By the beginning of the Cretaceous period, Pangaea was breaking up accompanied by increased plate tectonic activity. The proto Pacific, Indian and Atlantic oceans were formed at this time as spreading mid oceanic ridges produced large volumes of basaltic lavas causing a eustatic rise in sea level. The Tethys Ocean began to close in late Cretaceous times as the Arabian/Indian plates drifted northwards towards Eurasia.
The Cretaceous (145 to 65 Ma) is the longest geological period since the end of the Precambrian. The Weald of south east England provides one of the best areas for the study of Cretaceous strata.. Structurally, the Wealden uplift is due to the Alpine earth movements that produced an anticline which dips gently away from its east-west axis, although the fold also plunges to the west
The High Weald is formed of the Hastings Beds which are the oldest Cretaceous rocks in the centre of the anticline. The rocks are mainly sandstones that show cross stratification, ripple marks and plant remains. These were deposited in deltas within the shallow waters of the Wealden Lake. On the shores of the lake grew conifers, cycads and giant horsetails on which dinosaurs such as Iguanadon were feeding. The first remains of Iguanodon were discovered in 1822 by Mary Mantel in the Weald of Sussex. Ironstone nodules occur within clay bands providing a source of ore for the Wealden iron industry which developed in the 16th C to provide cannon for Tudor ships that were build of oak from the Wealden forests. The area was extensively forested until the demand for charcoal for smelting and timber for ships led to the removal of much of the woodland. Hammer ponds were used to power water wheels that operated bellows and hammers for the iron works. The High Weald was the centre of the iron industry in Britain until the beginning of the industrial revolution when Abraham Darby developed the coal fired blast furnace in 1709 in Coalbrookdale. There were also numerous small quarries that provided local building stone. Bateman's House, formerly Rudyard Kipling's home, is a good example of the use of local Ashdown sandstone. The High Weald forms a major watershed separating north and south flowing rivers including the Medway and the Wey, the Arun, the Ouse and the Cuckmere. River capture commonly occurs where some rivers cut back by headward erosion and divert the headstreams of others, thus increasing their drainage system.
The Low Weald is formed of Weald Clay and forms a horseshoe shaped outcrop around the High Weald. The clay vales are poorly drained (impermeable clay) and mainly provide pastureland. Brick making was based on the Weald Clay, particularly during the 19thC when bricks were in demand for the London market.
The Lower Greensand lies above the Weald Clay and it has been worn back by erosion to form prominent scarps overlooking the clay vales. Leith Hill on the northern escarpment forms the highest point in the Weald. The sandy acidic soils generally support heathland and coniferous woodland. A spring line marks the base of the greensand where water emerges along the junction with the clay. Note that the greensand is commonly orange or brown in colour due to oxidation, but the name came from greensand containing glauconite (hydrated potassium iron silicate) that outcrops on the Dorset coast. The two most prominent formations within the Lower Greensand are the Hythe beds (buff sandstones with chert layers) and the overlying Folkestone beds (orange/brown poorly cemented sandstones). The latter often show cross stratification; evidence of deposition in shallow seas. They also contain irregular contorted beds of ironstone (carstone) that were precipitated by ferruginous waters percolating through the sandstone after it was lithified. The Folkestone beds are quarried to provide soft building sand.
The Lower Greensand was deposited under shallow marine conditions as the Wealden Lake was invaded by the sea around 115 million years ago. Later as the sea deepened the Gault Clay was laid down. This clay is one of the most fossiliferous horizons in Britain containing a rich marine fauna of ammonites, bivalves and gastropods. It forms a narrow vale at the foot of the chalk escarpment. At Folkestone Warren rotational landslipping occurs in winter when the overlying chalk is saturated and slides over the impermeable Gault Clay. In 1915 a passenger train was derailed on the coastal railway line which was buckled by a landslide.
The Chalk encloses the Weald on three sides forming inward facing escarpments along the North and South Downs. The present river system was initiated on the chalk cover which has since been eroded over the Weald. The rivers have cut gaps through the chalk scarp; for example, the Wey gap at Guildford and the Ouse gap at Lewes. Note that where the dip of the chalk is steep as on the Hog's Back, the outcrop is narrow. Where the dip is gentle or horizontal, the outcrop forms extensive undulating downland. Dry valleys are common where the water table has been lowered. The Seven Sisters on the Sussex coast are dry valleys truncated by the sea. A spring line occurs at the base of the scarp along the junction between the chalk and the underlying Gault clay. Anglo Saxon settlements developed along the spring line. Villages with suffixes such as 'ham, ton and ing' date back to this period.
Geologically, the chalk is a fine grained white limestone formed from calcareous mud containing microscopic coccoliths derived from marine plankton. The Cenomanian transgression in late Cretaceous times covered much of southern England and since there was little sand and mud brought down by rivers, the sea remained relatively clear and the chalk sediment was free from impurities. Fossils are fairly common in the Lower Chalk including ammonites, belemnites, echinoids, bivalves and brachiopods. Micraster, the heart urchin is common as are brachiopods such as terebratulids and rhynchonellids. The Upper Chalk is characterised by the presence of flint nodules which may have been precipitated from silica rich ground waters percolating through the chalk. However, recent research suggests that the flint was formed by the sub surface breakdown of siliceous organisms such as sponges, radiolaria and diatoms during the deposition of the chalk.
Mass Extinction at the end of the Cretaceous
This event is known as the K-T extinction or the K-Pg extinction (Cretaceous-Tertiary or Cretaceous- Palaeogene). It occurred 66 million years ago when an asteroid some 10-15 kilometres in diameter impacted the Earth creating the Chicxulub crater in the Yucatan peninsula in the Gulf of Mexico. The boundary marking the extinction is formed of a thin layer of sediment that is rich in iridium which is abundant in asteroids and meteorites. The sediment represents the dust and shattered rock fragments produced by the impact. Shocked quartz which is produced by intense pressure, is also present in the sediment. Luis Alvarez, an Italian physicist, first proposed the impact hypothesis in the 1980s when he identified the iridium rich clay boundary layer near the ancient Umbrian town of Gubbio. Later the Chicxulub crater was discovered in the 1990s and it provided strong evidence in support of Alvarez's research.
However, it is likely that other events contributed to the mass extinction. In the Deccan plateau in India basalt lavas were erupted at the end of Cretaceous times. These would have produced vast amounts of CO2 and SO2 and contributed to global warming with acid rain killing off the vegetation. Whilst 75% of all species became extinct, the dinosaurs are often seen as the chief victims of catastrophic events, yet many invertebrates such as the ammonites had been in decline throughout the Cretaceous. Other creatures including bony fish and placental mammals developed during the Cretaceous, survived the K-T impact and then expanded in the Palaeogene.
John Downes
Geology / Geology Group Diary (34)
September 13, 2018, 05:16:37 pm
The Geology Group met at Merlin's Bridge village hall at 10.30am on Wednesday 12 September 2018. The topic for this month was BRITISH JURASSIC GEOLOGY.

A shallow sea on the western margin of the Tethys Ocean spread across Britain during the Jurassic Period (201-145 Ma) when the area lay between 30° and 40° N. A succession of alternating marine clays and limestones were laid down but due to several axes of uplift the Jurassic strata is thinner over the swells and much thicker in the intervening basins. The three most prominent uplifted areas of pre Jurassic rocks are the Mendip, Moreton and Market Weighton axes. Outcrops of Jurassic rocks extend in a diagonal belt from the Dorset coast up through the Cotswolds to the North Yorkshire Moors. In the early 19th C William Smith, a canal engineer, was the first to work out the Jurassic sequence of strata using ammonites as zone fossils. In 1815 he produced the first geological map of England and Wales.
The Lias clays are exposed in the cliffs around Lyme Regis and Charmouth in west Dorset. During the 19thC  quarrymen used the term Lias  (a corruption of the word layers) to describe the sequences of alternating thin limestones and bluish grey clays that reach a thickness of over 130 metres. They represent the sediments deposited on the margins of the Tethys Ocean which spread over much of the Permo-Triassic landscape of Britain. The Lias sea contained a rich fauna of ammonites and marine reptiles such as Ichthyosaurs and Plesiosaurs made famous by Mary Anning, the Victorian fossil collector who lived in Lyme Regis. Dark bituminous shales also occur in the Lias which suggest that the sea deepened from time to time. The shales contain iron pyrites, a mineral formed where there is a lack of oxygen in deep waters. The Bridport Sands occur at the top of the Lias sequence and mark a change in environmental conditions as sandy sediment was washed into the area.
Liassic sediments also outcrop along the coast between Staithes and Robin Hood's Bay in East Yorkshire.  They underlie the Cleveland Hills which form the northern part of the region bordered by Teesside to the north and the River Esk to the south. The Cleveland Ironstone Formation (Middle Lias) can be seen to the east of Staithes where it was worked in the 19th C for use in the iron and steel industry on Teesside. There are 4 main seams of bedded ironstone exposed on the coast and these can be traced inland where they thicken and outcrop in the escarpment around Guisborough. The harbour at Port Mulgrave was opened in 1856 to handle the output of the coastal mines. The iron ore occurs as an oolitic mudstone rich in siderite and chamosite and was deposited under shallow marine conditions; evidence includes a prolific shell fauna, ripple marks, trace fossils and cross stratification.
The Upper Lias contains the famous Jet Rock which is a bituminous shale horizon containing discrete  masses of the carbonaceous mineral jet. This appears to have been formed from drifted logs of wood which sank into the anaerobic muds on the sea floor and then became compressed under the weight of overlying sediments. The Whitby jet derives from Jurassic plants similar to the Chilean pine or Monkey Puzzle tree. Jet was worked by the Romans and was also popular in Victorian times particularly for mourning jewellery.
The Alum Shales occur at the top of the Upper Lias. These pyrite (FeS2) rich shales formed the raw material for the alum industry that began in the 17thC and continued up until the 1870s. The alum was used in tanning, dyeing and in medicine. The shales were quarried mostly on the cliff tops where the treated waste could be easily dumped on to the shore below. Calcination required alternate layers of brushwood and shale to be slowly heated over a period of a year or more. During calciation sulphuric acid was produced by the oxidation of the pyrite and it reacted with the aluminium silicates in the shale to form aluminium sulphate, The burnt shale was then steeped in water in tanks to extract the aluminium and iron sulphates from the shale. The resulting liquid was then concentrated by repeated boiling, evaporation and crystallisation of the salts. Whilst the iron salts remained in solution the  alum salts ( hydrated  sulphate of potassium and aluminium) would crystallise out. The exact time to cease heating was determined by floating an egg on the liquid!
The Lias of North Yorkshire represents repeated cycles of marine sedimentation, each cycle containing clays, shales, sandstones and ironstones in upward succession. As the cycle coarsens upwards, the water becomes shallower and oxygenation increases giving rise to a profusion of marine creatures including ammonites, belemnites, bivalves and marine reptiles. Robin Hood's Bay is a wonderful fossil hunting locality. Dactylioceras and Hildoceras are two well known ammonites from the Upper Lias, the latter is named after St Hilda who founded Whitby Abbey and is reputed to have turned the local sea serpents into stone which are now found as ammonites!
MID JURASSIC (174-163 Ma)
The Cotswold Hills overlook the low undulating Vale of Severn that is punctuated in places  by small hills known as outliers, the most famous of which is Bredon Hill. Outliers are small areas of newer rock surrounded by older strata; for example, Bredon Hill is capped by Inferior Oolite but surrounded by Lias Clay. An outlier is really an isolated outcrop left behind as the main escarpment gradually retreats under the influence of weathering and erosion. A closer look at Bredon Hill reveals that it has a steep scarp to the north and the strata dip south with a strong fault along its southern margin. Notice the stepped profile, well seen on the eastern side of the hill. This bench feature is produced by the resistant marlstone, a ferruginous sandstone at the top of the middle Lias.
The Cotswold escarpment is formed mostly of limestones of the  Inferior Oolite Group that are formed of tiny ooliths consisting of concentric layers of calcium carbonate that has been deposited by circulating currents in shallow shelf seas (similar to the Bahamas shelf today). The colour of the limestone varies from pale buff to orange brown depending on the concentration of iron minerals in the cement. Leckhampton Hill near Cheltenham shows a section through the Inferior Oolite. The main limestone is well bedded and jointed and so makes an excellent building stone since it can be cut easily into blocks; hence it is referred to as a freestone. Much of Regency Cheltenham was built of stone from the Inferior Oolite. The Devil's Chimney at Leckhampton was left behind by the quarrymen in the 1780s to form a local land mark. Crickley Hill provides an example of a promontory on the Cotswold scarp which it is cut by a deep re-entrant valley followed by the A417 Gloucester road. The cliffs along the face of Crickley hill are formed of the Pea Grit; this is a pisolitic limestone where the grains are much larger than the ooliths in the freestones. Clypeus (echinoid), Trigonia (oyster) and rhynconellids (brachiopods) are common fossils. Highly developed ammonites with intricate suture patterns are also found in the Inferior Oolite.
The Great Oolite Group  overlies the Inferior Oolite and is found mainly on the dip slope of the Cotswolds. Near the base of the Great Oolite is a famous bed called the Stonesfield Slate. This is not a slate in the geological sense but a sandy limestone that is thinly bedded and so forms flagstones 2-3 cm thick. These were extensively quarried for roofing tiles in the 18C & 19C and can be seen on old cottages and farm buildings throughout the Cotswolds today. However, as long ago as 2000 BC in the Neolithic period  the 'slates' were used to build the entrance to the Bellas Knap long barrow! But the Stonesfield Slate was made famous by William Buckland, the great Oxford geologist who first identified the remains of the carnivorous dinosaur  Megalosaurus bucklandii in 1827. There are also leaf impressions of cycad leaves indicating that a rich vegetation must have bordered the coastal lagoons in which the sandy limestones were deposited.
At the top of the Great Oolite is the White Limestone Formation which can be seen in the old cement works quarry at Kirtlington, a few miles west of Oxford. Here the limestone is hard, white and shelly. The most abundant brachiopod is Epithyris oxonica which makes up much of the rock. However the quarry has also yielded the bones of pterosaurs (flying reptiles), plesiosaurs (marine reptiles) and early mammals.
The Cotswolds provide a good example of scarpland topography where relatively resistant limestones alternate with softer clays and shales. The clay vale, the scarp and the dip slope are the three essential features of this type of topography. Note that the strata in the Cotswolds dip gently south east and young in the direction of dip. Since the limestone is permeable, water percolates down to the level of the underlying impermeable clay thus producing springs at the junction of the two rocks. Many villages developed in Anglo Saxon times along the spring line at the foot of the Cotswold scarp. Dry valleys are also common in the limestone since the water table has been gradually lowered as the scarp has receded
The Dorset coast between Weymouth to Swanage provides some excellent exposures of Upper Jurassic rocks. The Oxford Clay outcrops in the core of the Weymouth anticline but it extends in a broad swathe  north west through Oxford to Lincolnshire and beyond. It has yielded aquatic reptilian remains, and a variety of fossil ammonites, belemnites and brachiopods.  Economically it is important as a brick making clay which is high in carbon content thus reducing firing costs. Fletton bricks from Peterborough were used in building much of 19th C London.
On the coast around St Alban's Head the Kimmeridge Clay appears below the Portland Stone cliffs. Kimmeridge Bay is the type locality for this formation, consisting mainly of black shales and clays with bands of limestone that form the Kimmeridge ledges. Some of the shales are bituminous and combustible; in fact, the Kimmeridge beds are an important source rock for petroleum under the North Sea. Wytch Farm on the Isle of Purbeck is the largest onshore oil field in Britain
The Isle of Portland forms the southern flank of the Weymouth anticline and it is highest in the north and dips gently down to sea level at Portland Bill in the south. Although the Portland Stone extends across most of the area 75% of it is covered by the lower Purbeck Beds (quarry overburden). The Portland Stone is a fine grained white limestone that is well jointed and easily dressed into rectangular blocks. It became popular as a building stone in the 17th C when St Paul's Cathedral was rebuilt after the Great Fire of London. Most of the quarries are on or near the coast so that the stone could be exported by sea. A fossiliferous limestone known as the Roach occurs at the top of the Portland Stone and contains 'Portland screws and osses 'eds ' otherwise known as Aptylexia (gastropod) and Laevitrigonia (bivalve). Large ammonites such as Titanites have been extracted from the Portland beds.
The coast from Durdle Door to Lulworth Cove and Mupe Bay is formed of a wall of Upper Jurassic strata that create a barrier protecting the softer Wealden Beds (Lower Cretaceous) from the continuous erosion of the sea which has already broken through in several places. Stair Hole adjacent to Lulworth Cove provides a good example of the way that the sea is excavating the soft Wealden beds behind the Portland/Purbeck wall. The famous Lulworth crumple in the Purbeck beds shows the effect of the Alpine earth movements during the Oligocene. Lulworth Cove itself has been cut back through the sands and clays of the Wealden beds to the high cliffs of the massive Chalk ridge that extends eastwards into the Purbeck hills. Note the exposure of Upper Greensand (at the base of the chalk) near where the road reaches the cove and here the greensand really is greenish in colour due to the presence of the iron mineral glauconite. On the east side of Lulworth Cove is a fossil forest that consists of silicified boles which mark the base of conifers that grew during Purbeck times.
On the Isle of Purbeck the most important structural feature is the Purbeck monocline produced by the northward thrust of the Alpine orogeny. The chalk ridge of the Purbeck hills can best be seen around Corfe Castle that stands on a knoll between a twin water gap. Here the chalk is almost vertical hence the width of the Purbeck hills is relatively narrow. (Note the rule of thumb is that the steeper the dip of a stratum, the narrower the outcrop). The underlying Wealden, Purbeck and Portland beds all dip steeply north near Corfe but then become horizontal across the Isle of Purbeck forming the southern limb of the monocline. The best place to see the actual curvature of the monocline is in St Oswald's Bay to the west of Lulworth.
Swanage was the centre of the Purbeck stone trade in the 18th & 19th C but in Medieval times Purbeck 'marble' was much in demand  for cathedral interiors, eg.Westminster Abbey and Salisbury Cathedral. A beautiful example of 'marble'columns can be seen in Eldon Memorial Church near Kingston.
John Downes
Geology / Geology Group Diary (33)
July 12, 2018, 02:19:59 pm
The Geology Group met at 10.30am on Wednesday 11 July at Merlin's Bridge village hall. This month's topic was...
The Permian Period lasted from 290 to 245 Ma which brought to an end the Palaeozoic era. The period was terminated by a major catastrophic event known as the Permian Mass Extinction when 95% of all animals and plants became extinct.  Invertebrate groups such as trilobites, goniatites, rugose corals and many brachiopods became extinct. On land many amphibians and reptiles died out to be replaced by the early dinosaurs in the succeeding Triassic Period (245-208). A combination of various factors may have brought about the great extinction. Here are some of the possible events. 1.Asteroid impact; dust and rock fragments from the collision of an asteroid 100 km in diameter would blot out sunlight for years. However, it is difficult to find evidence of such craters since the earth's surface has changed considerably since Permian times. 2.Formation of Pangaea; this super continent emerged during the Permian causing a eustatic (world wide) lowering of sea level and marine regression over the continental shelves thus destroying shallow water marine habitats. Polar glaciations would also contribute to lowering sea level. 3.Vulcanism; the vast Permian basalt lava flows in Siberia would have been produced by powerful eruptions that would emit CO2 and SO2 in enormous quantities causing climate change and the destruction of vegetation and consequent death of animal life.  4.Climatic events; Global warming due to enhanced greenhouse effect would cause increased aridity (continental climate) and salinity (high evaporation), severe decline in oxygen levels and the presence of excess carbon dioxide, methane and hydrogen sulphide in the oceans would poison marine life.
The Permo-Triassic Environment.
[The Permo-Triassic rocks are often referred to as the New Red Sandstone]
As the Rheic Ocean closed and the Variscan Mountains were formed, the supercontinent of Pangaea developed as the existing continents collided. Britain was located between 20° and 30°N on the north west side of the Tethys Ocean. At the beginning of Permian times Britain was an arid upland area where barchan sand dunes accumulated under the influence of the north east trade winds. Fault bounded basins such as the Vale of Eden received great thicknesses of aeolian (wind blown) sediment at this time. In the late Permian the marine transgression of the Zechstein Sea covered much of NE England and the North Sea area. The sea initially covered the desert landscape with muddy sediments forming the Marl Slate, which is a bituminous shale containing fish remains. Next the Magnesian limestone was laid down and this now forms a well defined escarpment stretching from Sunderland to Teesdale. This limestone is formed of the mineral dolomite or calcium magnesium carbonate. It contains many marine fossils such as brachiopods, gastropods and bivalves. A concretionary form of the Magnesian limestone known as the 'cannonball limestone' is well developed in the Roker area of Sunderland. As rapid evaporation occurred in the hot arid climate within the Zechstein Sea, a sequence of repeated cycles of desiccation occurred in which salts were precipitated in order of increasing solubility. Starting with the least soluble calcium carbonate CaCO3,then gypsum CaSO4, halite NaCl, magnesium and potassium salts (most soluble). The fluctuating level of the Zechstein Sea would account for the repeated cycles of desiccation. In the 1970s the Boulby mine near Staithes (Cleveland) was opened in order to extract potassium chloride (KCl)  and halite (NaCl) from up to 1500 metres below the surface. The potash is required for agricultural fertilizer; rock salt is also produced and both minerals are exported via Teesside. North Sea oil and gas reservoirs are found in Permian sandstones trapped by impervious salt domes. The natural gas derived from underlying Coal Measures, migrated upwards through the Permian sands.
The Triassic is so called from its threefold division in Germany, but in Britain the middle division (Muschelkalk) is missing. Here arid conditions continued throughout the period, beginning with the Bunter Sandstone (Buntsandstein) and Pebble Beds, now part of the Sherwood Sandstone Group. Braided rivers and flash flooding transported coarse sands and gravels from the Variscan highlands to produce cross stratified sandstones, pebble beds and conglomerates that are characteristic of the Lower Triassic strata. Examples of the pebble beds can be seen in Staffordshire on Cannock Chase, High Shutt near Alton Towers and in Park Hall Country Park. The Budleigh Salterton Pebble beds in South Devon also show evidence of braided rivers and wind faceted 3 sided driekanter pebbles on desert surfaces. The overlying Keuper Marl now called the Mercia Mudstones comprise red-brown clays up to 1000 metres thick in the English Midlands and they contain considerable deposits of halite and gypsum. These were laid down in shallow playa lakes as evaporites. Note that rock salt was extensively worked around Northwich and Droitwich and gypsum was mined in the Newark area. At the end of the Triassic, the Rhaetic marine transgression heralded the onset of the Jurassic period as the shallow marginal waters of the advancing Tethys Ocean covered the desert plains and playa lakes. The Rhaetic bone bed represents a condensed deposit of fish teeth, scales, and bones, coprolites from aquatic reptiles and early dinosaur bones that may be the result of mass mortality due to salinity changes or simply the sweepings of  the advancing seas.
Commercially the Sherwood Sandstone Group is a major aquifer providing water for both domestic and industrial users throughout the Midlands. Where it is sandwiched between the underlying impervious Upper Carboniferous strata and the overlying Mercia mudstones, the porous and permeable Sherwood sandstone ( up to 600 metres thick) can yield up to 125 litres per second from a vast underground reservoir.  Coarse grained sandstone is both highly permeable and porous since there are spaces between the grains which allow water to pass through and also be stored.
"To see the world in a grain of sand.........(and) hold infinity in the palm of your hand." William Blake
John Downes
We gathered at Stepaside Ironworks [SN141074] at 10.30 am on the last day of flaming June  with shorts and sunhats at the ready. Our first task was to identify the building stones used in this magnificent Victorian edifice. The orange brown colour of the stonework reflected the iron rich Coal Measure sandstone and some blocks glinted in the sunlight where quartz crystals had developed within joints in the sandstone; muscovite mica flakes could also be recognised on some surfaces. When we reached the wall of the casting house there was ample evidence of iron smelting in remnants of slag where we discovered fragments of ironstone, anthracite and limestone.
This site of industrial archaeology reflects the importance of economic geology in the 19th century in South Pembrokeshire. All the materials for iron making were available locally; anthracite, iron ore and limestone, and the Saundersfoot Railway had already been extended to Stepaside when the ironworks opened in 1849. However, supplies of bedded iron ore from the coalfield were limited and as economic conditions declined, the ironworks was forced to close in 1874. Today the ruined buildings have been restored and developed as a tourist attraction.
The stone casting house with its three arched doorways and circular openings stands as a monument to solid Victorian engineering. The two blast furnaces originally stood in front of the furnace wall, one to each pair of arches. The furnaces were loaded by overhead bridges leading from the upper platform where the raw materials were assembled. Before the furnaces were charged, the anthracite was converted into coke in the coke ovens and the iron ore was roasted in the calcining kilns. Compressed air to aid combustion was provided by the blowing house, which is the tall building to the right of the furnace wall.

After exploring the ironworks, we climbed up the pathway to the remains of Grove colliery. This mine was opened in 1853 to supply anthracite from the Kilgetty Vein. A branch line from the Saundersfoot railway linked the colliery to the ironworks. Today you can still see the ruins of the winding engine house that lifted the cages up and down the shaft, and the Cornish pumping house that contained a 274 hp beam engine that raised water from the mine

From Stepaside we drove across to Pendine for lunch; this seaside resort  was crowded with holiday makers and those attending the motor rally on Pendine Sands. However, we managed to leave the crowds behind as we traversed westwards from Dolwen Point [SN234079] to examine the Carboniferous limestone succession that forms the northern boundary of the Pembrokeshire and South Wales coalfields. The base of the limestone is seen at Dolwen Point and the strata dips south west for about 1 km to Gilman Point where the top of the succession is exposed. At Dolwen Point the Avon Group (Lower Limestone shales) is composed of well bedded and jointed grey limestones in the centre of which is a thick bed of calcareous mudstone. The mud has undergone soft sediment deformation induced by gravity on a gently inclined depositional slope. There are slump folds, convolute beds, pillow beds and fractured blocks that have sunk into the mud; the mudstones are sandwiched between more competent limestone beds.
The Abercriban Oolite overlies the Avon Group near the cliff steps just west of Dolwen Point. This is a massively bedded bioclastic limestone where we examined fallen blocks containing shell beds and lenses with productid brachiopods (Dictyoclostus), single corals (Zaphrentis) and colonial corals (Lithostrotion junceum).  We then made our way across the sands to Gilman Point where the massively bedded Dowlais Limestone is exposed. On the western side of the headland the limestone is overlain by cemented periglacial head deposits up to 10 metres thick containing large clasts of quartzite and limestone. This material appears to have slipped down the side of a preglacial valley having been lubricated by surface meltwater as the frozen ground began to thaw at the end of the Devensian glaciation in a process known as solifluxion. On returning along the beach we discovered some remains of a submerged forest. The submergence relates to the Flandrian marine transgression at the end of Pleistocene times.
John Downes
Geology / Geology Group Diary (32)
June 14, 2018, 06:27:21 pm
The Geology Group met at 10.30am on Wednesday 13 September 2018 at Merlin's Bridge Village Hall. The topic for the meeting was:
By the beginning of the Namurian (325-315 Ma) much of Northern England had became a region of swampy delta plains and shallow lakes, with large rivers draining south off the northern ORS continent. The muds, sands  and coal seams were laid down in rhythmic cycles of sedimentation (cyclothems) as sea level fluctuated throughout the Upper Carboniferous. The Millstone Grit so characteristic of the Pennines, represents major deltaic development during the Namurian. And by Westphalian times (315-300 Ma) delta top swamp forests flourished intersected by numerous distributary rivers. The typical Coal Measure cyclothem of shale, sandstone and coal seam represents the changing environmental conditions ranging from the marine submergence of the delta plain, through the development of thick deltaic muds and sands to the growth of swamp forest on the delta top.
South of St Georges Land, the Rheic Ocean was closing with the resultant formation of the Variscan Mountain ranges that stretched from the Appalachians through the Pyrenees and Atlas Mts to the Urals in Russia. This Variscan Front can be seen in SW England where the Culm Measures (sandstones, mudstones and thin coals) were deposited in a subsiding basin (Devon syncline). Variscan folding and thrust faulting at the end of the Carboniferous can be seen particularly well in the South Wales and Pembrokeshire coalfields. The coastal sections at Broad Haven and Saundersfoot show intense thrusting and folding within the Coal Measures and the chevron folds at Millook Haven in North Devon are classic Variscan features. The intrusion of the granite batholith in Cornwall and South Devon (Dartmoor, Bodmin Moor,etc) is associated with the Variscan earth movement as pressure built up from the south.
In late Carboniferous times sheets of molten magma were injected into the rocks of the northern Pennines. One of the most famous of these intrusions is the Whin Sill, a 30 metre thick dolerite sill that covers an area of over 500 km2 yet it is exposed at the surface only as a narrow outcrop from the Farne Islands, through Northumbria to Cross Fell on the Pennine escarpment and in Teesdale (High Force waterfall). Hadrian's Wall follows part of the outcrop of the sill which provides an excellent defensive feature.
In the Midland Valley of Scotland there was considerable vulcanicity throughout Carboniferous times. The Clyde Plateau basalts are up to a 1000 metres thick and Arthur's seat in Edinburgh represents an extinct Carboniferous volcano. Salisbury Crags is a massive sill where James Hutton proposed his ideas on the origin of igneous rocks in the late 18th C. 
THE PENNINES  In the Peak District and  the Yorkshire Pennines the Millstone Grit forms dramatic escarpments where it has been removed by erosion from above the underlying shales and limestone as for example on the Roaches, on Stannage Edge, on Kinderscout or on Ilkley Moor. The term 'grit' is commonly used but it is actually a coarse sandstone that was employed extensively in the 19th C to build the 'dark satanic mills' and the back to back terraced houses of industrial Yorkshire. The dark exterior colour of Millstone Grit is largely due to industrial pollution since in fresh specimens the rock is a light colour due to the abundance of quartz. The gritstone bands alternate with shales and thin coal seams in a series of cyclothems. Note that cross stratification is often seen in the gritstones, evidence of their deltaic origin.
The Millstone Grit passes conformably into the Coal Measures which outcrop on the flanks of the Pennines. On the Yorkshire coalfield the strata dips gently eastwards so that the exposed coal measures are eventually concealed beneath a cover of younger Permo-Triassic rocks. Here the cyclothems are fully developed with some seams reaching up to 3 metres in thickness. During Victorian times the exposed field fuelled the industrial revolution, but as these seams were worked out deeper pits were sunk down to 800 metres in the concealed field. Kellingley Colliery in North Yorkshire was a deep mechanised pit developed in the 1960s to feed the Ferrybridge Power station, but the decline of coal fired power stations and the uneconomic cost of deep mining, resulted in the mine was closure in 2015...it was the last deep coalmine in the UK.

We assembled at Freshwater West car park at 10.30am. 14 members were present. A short walk brought us to Little Furzenip headland close to the B4139 road.  From here we had an excellent view of the rock platform exposed at low tide. The steeply dipping Old Red Sandstone sequence youngs to the south forming the southern limb of the Castlemartin-Corse anticline. The foreshore is bisected by the Flimston Bay Fault, a Variscan dextral wrench fault with a displacement of about 120 metres. This fault is a major tectonic feature extending southwards through the Pembroke peninsula, across the Bristol Channel and through Devon where it is known as the Sticklepath Fault.
From Little Furzenip we followed the track down to the beach where the ORS strata was exposed in a series of isolated outcrops to the east of the Flimston fault. Here we looked at conglomerates, sandstones and calcrete bearing mudstones usually arranged in fining upward sequences each with an erosion surface at the base of the unit followed by lag gravel, coarse sandstone and then grading upwards into fine sand and mudstone. This graded bedding indicated the way up of the rock sequence. We also examined some beautifully displayed ripple drift bedding sequences. These are typical of highly sinuous meandering streams that must have flowed across the arid alluvial plains in Devonian times.  On the eastern side of the fault gap at Little Furzenip there are thich layers of calcretes. These represent fossil soils produced where alternating wet and dry seasons result in leaching followed by evaporation and the precipitation of calcareous minerals within the soil.  The multiple calcrete  profiles at Freshwater West are of regional importance and are known as the Chapel Point Calcretes from the type area on Caldey Island.
After a rather slow lunch at the Hibernian Pub we assembled on the front in West Angle Bay and observed the line of the synclinal axis that trends WNW-ESE through the bay. We noted the dark grey limestones and shales of the Avon Group (formerly known as the Lower Limestone shales) at the base of the Carboniferous Limestone sequence that outcrop on either side of the bay with the younger Black Rock Limestone occupying much of the area beneath the sandy beach. Next we walked along the foreshore to the first cove that lies below a ruined limekiln on the north side of the bay. Here we saw the Avon Group strata dipping towards the south and resting discordantly upon the almost vertical beds of Black Rock Limestone.  There has been a strong northerly thrust movement pushing the Avon Group rocks northwards over Black Rock Limestone. The line of this thrust fault can be traced through Cove 2 and into the south side of Cove 3 where a narrow zone of brecciated limestone marks the lower side or footwall of the thrust. These broken and shattered rocks are the result of movement along the thrust fault during the Variscan earth movements.
After a rather slow lunch at the Hibernian Pub we assembled on the front in West Angle Bay and observed the line of the synclinal axis that trends WNW-ESE through the bay. We noted the dark grey limestones and shales of the Avon Group (formerly known as the Lower Limestone shales) at the base of the Carboniferous Limestone sequence that outcrop on either side of the bay with the younger Black Rock Limestone occupying much of the area beneath the sandy beach. Next we walked along the foreshore to the first cove that lies below a ruined limekiln on the north side of the bay. Here we saw the Avon Group strata dipping towards the south and resting discordantly upon the almost vertical beds of Black Rock Limestone.  There has been a strong northerly thrust movement pushing the Avon Group rocks northwards over Black Rock Limestone. The line of this thrust fault can be traced through Cove 2 and into the south side of Cove 3 where a narrow zone of brecciated limestone marks the lower side or footwall of the thrust. These broken and shattered rocks are the result of movement along the thrust fault during the Variscan earth movements.
Next we clambered over the rocks into Cove 3 where there are several interesting geological structures on the foreshore including two periclinal folds that appear as shallow elongated domes rather like the upturned hull of a boat. The axes of the folds are orientated parallel to the main synclinal axis of West Angle Bay. The rocks are also cut by a series of en echelon veins that are where the rock has been sheared and the resulting tension gashes have later been filled with the mineral calcite. You may also notice some small circular holes with radiating fractures about 30 cms long. These are not natural features, rather they result from blasting operations during quarrying many years ago. Many of the limestone faces contain fragmented brachiopods, corals and crinoids and some calcareous mudstones are bioturbated showing infilled burrows. Some of the latter were preserved in chert that had been precipitated from silica rich waters. The burrows are referred to as trace fossils since the original organism that produced them has long since been destroyed. Finally we reached the far northern side of the cove where the Avon Group mudstones were faulted against the limestone. This is a normal fault with a downthrow to the south. The mudstones are orange brown due to downwash from the overlying iron rich glacial drift. Note that the rock fragments (clasts) in the drift are very angular since they have been shattered under periglacial conditions.                  John Downes
Geology / Geology Group Diary (31)
May 09, 2018, 08:13:20 pm
The Geology Group met at Merlin's Bridge Village Hall at 10.30 am on Wednesday 9 May 2018. The topic for the meeting was
By the end of Devonian times the Caledonian mountains had been eroded down to lowlands and the Old Red Sandstone continent was inundated by the shallow shelf seas of the Rheic Ocean that separated Avalonia from Gondwana. This ocean was closing during the Carboniferous and continental collision would produce the Variscan Fold mountains extending from the Appalachians through the Pyrenees and the Atlas to the Urals.
During the Lower Carboniferous Period (363-325 Ma) Britain lay in the equatorial region and so experienced hot wet tropical conditions suitable for the growth of coral reefs on the margins of the seas in which carbonates were deposited on offshore ramps. St George's Land  formed a land barrier stretching from Wales across to Belgium; whilst marine limestones were laid down in the Mendips and South Wales, across much of Ireland  and in the Pennines to the north. The shallow shelf seas were rich in marine life particularly corals such as Caninia (single coral) and Lithostrotion (colonial coral) and numerous brachiopods including Productus and Spirifer. Crinoids also grew in profusion. In North Yorkshire, Northumberland and the Midland Valley of Scotland deltaic sediments were deposited from rivers flowing off the northern landmass.
South West England.
The Mendip Hills lie to the south of Bristol and extend WNW-ESE from Weston super Mare to Frome in Somerset. This axial trend reflects the influence of the Variscan earth movements that produced a major asymmetrical anticlinal structure plunging eastwards. Since the main thrust came from the south as the Rheic Ocean closed, the dips on the north side of the Mendips are much steeper than those on the south. Since the oldest rocks lie in the core of an anticline we find the Upper Old Red Sandstone in the centre of the Mendips where it forms the upland known as Black Down. The Carboniferous Limestone has been eroded over the centre of the anticline and now outcrops on the flanks of the Mendips. The northern outcrop is well seen in Burrington Combe which is one of several valleys cut into the limestone. However, during Triassic times flash floods would have dumped vast quantities of limestone debris and sand into these valleys. Today erosional remnants of the Dolomitic Conglomerate (which in fact is a breccia) can be seen resting unconformably on the sides of Burrington Combe. Thus much of the Mendips have been exhumed from beneath a Triassic cover.  The steeply dipping limestone beds are well seen at the Rock of Ages which is where the Rev.Montague Augustus Toplady is said to have sheltered in a cleft during a storm in 1763. According to the apocryphal story he received divine inspiration after which he wrote the classic geological hymn 'Rock of Ages'! Aveline's Hole is a fine example of a swallow hole that follows the inclined bedding plane and leads down to two large chambers where the remains of a Mesolithic cemetery have been found.
Cheddar Gorge on the south side of the Mendips was cut by meltwater during cold periglacial phases of the Pleistocene Ice Age when permafrost would render the limestone impermeable. In warmer interglacial times the waters would descend into underground passages and caves leaving the gorge dry, as it is today. The caves contain the remains of animals that lived in Devensian times and the bones of 'Cheddar man' in Gough's Cave have been dated to around 7150 BC.
Ebbor Gorge lies about 8 kms to the south west of Cheddar where the Ebbor Thrust has forced the Carboniferous Limestone over Namurian quartzite that is exposed by a stream near the entrance to the gorge. The thrust zone extends along the SW margin of the Mendips and demonstrates the powerful effect of the Variscan earth movements. Wookey Hole near Wells is a magnificent swallow hole leading to some 25 underground caverns through which flows the River Axe. Human and animal remains indicate that the caves have been used since Palaeolithic times. Wookey Hole is a SSSI but like Cheddar, it is highly commercialised and a tourist' honey pot'.
The Avon Gorge which is spanned by the Clifton Suspension Bridge, cuts through a complete sequence of Lower Carboniferous rocks. In 1905 Vaughan pioneered the use of an assemblage of brachiopods and corals to create fossil zones K,Z,C,S & D to subdivide the strata [Cleistopora, Zaphrentis, Caninia, Seminula,& Dibunophyllum]. Today these zones have been superceeded by a classification based on 6 cycles of marine sedimentation during the Lower Carboniferous.
The Pennines
Whilst there is a conformable transition from the ORS into the Lower Carboniferous strata to the south of St George's Land; to the north there is a marked unconformity between the Carboniferous and older rocks. This suggests that whilst the sea advanced steadily in the south providing an unbroken record of sedimentation; in the north uplift of the basement blocks (Askrigg and Alston blocks) meant that the sea did not cover them until midway through the Lower Carboniferous. Only in the basin areas between the blocks was there continuous cyclic sedimentation of Yoredale mudstones, sandstones and thin limestones.
The West Riding of Yorkshire includes most of the Central Pennines that are dissected by several rivers which are tributary to the River Ouse. These rivers flow through and form the Yorkshire Dales from Swaledale in the north; Wensleydale; Nidderdale; Wharfedale; Airedale to Calderdale in the south. For the most part the rocks are Carboniferous Limestone, and Yoredale beds and they form a faulted tilted structure known as the Askrigg Block formed of Lower Palaeozoic basement rocks.. The block dips gently north eastwards but the higher western side is marked by steep scarps along the line of the Dent Fault and the South Craven Fault. The maximum upthrow along the Craven faults is about 1600 metres. Giggleswick Scar (NW of Settle) is a good example of a fault scarp along the line of the South Craven Fault. The younger Namurian gritstones of the Craven lowlands lie on the downthrow side of the fault whilst the older Carboniferous limestone forms the upthrow side of the fault.
Malham Tarn also rests on impervious Silurian slates, but notice that its outfall disappears underground when it reaches the limestone at the North Craven Fault. Malham Beck issues as a Vauclusian Spring at the base of Malham Cove, a natural amphitheatre with a well developed limestone pavement above it. The clints (blocks) are cut by rectilinear grykes (fissures) that form as solution weathering widens the joints in the horizontally bedded limestone. Cawden Hill near Malham village is one of several reef knolls that lie along the southern margin of the Mid Craven Fault. They are formed of calcite mudstones with some shelly limestones rich in corals, crinoids and brachiopods. These reef knolls clearly formed  on the margins of the Lower Carboniferous seas where the water was shallow above the Askrigg Block. Another spectacular karstic feature is Gaping Ghyll, a large swallow hole that takes the water of Fell Beck off the slopes of Ingleborough. This peak is capped by impervious Millstone Grit and Yoredale beds but as the water drains off on to the underlying limestone it disappears down numerous sink holes (swallow holes) including Gaping Ghyll which has the highest underground waterfall in Britain (98 metres).
The strata in the Yorkshire Dales are gently dipping to the northeast so the Great Scar Limestone is well exposed in the Craven District but farther north the overlying Yoredale beds are more common. Yoredale is an older name for Wensleydale where alternating thin limestones, shales and flagstones occur in rhythmic succession. Such cycles of sedimentation reflect the fluctuating sea levels above the Askrigg Block towards the end of Lower Carboniferous times. Differential erosion of the strata on the valley sides has produced a stepped topography with the limestone forming prominent terraces. Also where the rivers cross the harder rocks there are often spectacular waterfalls. Hardraw Force can be visited through the Green Dragon Inn in Wensleydale and lower downstream on the River Ure are a series of rapids known as Aysgarth Falls.
Swaledale was a major lead-zinc mining area reaching peak production in the late 18th C. The most important mineralised zone occurred on the north side of Swaledale, particularly around Gunnerside Gill. The veins run approximately east-west along pre existing faults in which hydrothermal minerals were precipitated. Gangue minerals such barytes and fluorite also occur in the veins and some old mines have been reworked in recent years in order to obtain these minerals. The source of the hydrothermal fluids is considered to be a large granite intrusion deep below the Askrigg Block and the rising granite also provided the necessary buoyancy to elevate the block above the surrounding lowlands that occupy 'basin' structures where great thicknesses of sediment accumulated in Carboniferous times in contrast to the relatively thin cover over the Askrigg Block.          John Downes 

Geology / Geology Group Diary (30)
April 12, 2018, 09:10:21 pm
The Geology Group met at 10.30am on Wednesday 11 April 2018 at Merlin's Bridge Village Hall. This month's topic was the
When the Iapetus Ocean finally closed at the end of the Silurian Period continental collision between Laurentia, Avalonia and Baltica produced the Caledonian Mountains along a suture line that extended from Greenland and Scandinavia through Scotland and Wales, to Newfoundland and New England. Since these continental areas were situated near to the equator and cut off from rain bearing maritime influences, the climate became arid with low humidity and rivers fed by torrential storms often produced flash flooding. The rapidly eroding mountains provided a source of coarse sediments that were deposited in lakes and rivers in the  inter- montane basins; these muds, sands and gravels eventually formed the rocks of the Old Red Sandstone. These eroded continental deposits were first laid down in Scotland in mid Silurian times but in the Brecon Beacons in South Wales deposition of the ORS did not begin until the end of the Silurian. However, whilst continental conditions continued throughout the Devonian Period (409-363 Ma) over most of Britain, a shallow shelf sea covered South Devon on the northern margin of the Rheic Ocean that separated the ORS landmass from Gondwana.
South West England. It is of interest to note that the Devonian Period was named by Sedgwick and Murchison in 1839 when they were working on the marine sediments in Devon and Cornwall. It was later realised that the continental ORS was of a similar age since in North Devon it is interbedded with Devonian marine sediments. The Foreland Grits (basal ORS) form steep cliffs near Lynmouth but these are succeeded by marine shales and then the Hangman Grits (Middle ORS)  east of Combe Martin. Around Ilfracombe to the west, marine sediments outcrop but these are overlain by an Upper ORS outcrop that extends to the coast near Morte Point. In South Devon the Torquay Limestone is a shelf sea deposit formed of reefs containing corals, brachiopods and gastropods. In North Cornwall marine mudstones have been converted into slates by the Variscan orogeny and the famous 'Delabole butterfly' is an example of a brachiopod Spirifer verneuili that has been deformed by intense earth movements.
South Wales and the Welsh Borders. A large triangular outcrop of ORS extends from Shropshire south to the Severn estuary and west through the Brecon Beacons to Pembrokeshire. Up to 3000 metres of mudstones, sandstones and conglomerates accumulated on river flood plains and coastal lowlands bordering the Rheic Ocean. In the semi arid tropical climate thick beds of calcretes formed as evaporation brought carbonate minerals to the surface. Spore bearing plants grew near the rivers where primitive fish were living. The well known Ludlow Bone bed at the base of the ORS in Shropshire contains fish scales and spines in a condensed sequence only 4 cm thick. Cephalaspis is a typical armoured jawless fish found in the Lower ORS of the Welsh borders. In Pembrokeshire at Red Cliff, Marloes, there is a conformable junction between the Gray Sandstone Group ( Silurian) and the Lower ORS. Thick calcrete beds are present at Freshwater West and Chapel Point, Caldey Island.
Scotland. The ORS derived from the eroding Caledonian mountains, covers large areas in Scotland.  In the Midland Rift Valley up to 9000 metres of sediment was deposited by  braided streams rushing down from the surrounding highlands and forming alluvial fans that spread out at the foot of the valley sides (Highland Boundary Fault & Southern Uplands Fault). Extensive vulcanism also occured as evidenced by the basalt lava flows inter bedded with the Lower ORS sediments. The Ochils and Sidlaw hills are formed of these lavas
In NE Scotland the Orcadian Basin covered the area around the Moray Firth through Caithness to the Orkney Islands. This was the site of several large shallow lakes surrounded by alluvial plains and along the edge of the highlands alluvial fans developed where rivers deposited thick beds of sand and gravels. The Old Man of Hoy is a much photographed sea stack on Orkney where the ORS cliffs rise sheer from the sea. On the mainland the Caithness Flagstones are one of the most famous rock formations within the Middle ORS. They consist of banded and varved (seasonal deposits) lake sediments; rhythmic alternations of mudstones, dolomitic limestones and cross stratified sandstones. The carbonate horizons contain beautifully preserved freshwater fish remains, particularly in the Achanarras fish band. In 1839 Hugh Miller, a Cromarty quarry worker, discovered calcareous nodules within the ORS that contained armoured jawless fish like Pterichthys.  He spent many years collecting fish specimens for local museums and in 1847 published an account of his work entitled 'The Old Red Sandstone'. A small outlier of Middle ORS occurs at Rhynie in Aberdeenshire where volcanic waters rich in silica entered a peat bog and preserved a remarkable array of primitive vascular plants in chert.
Evolution at the cross roads?  In Devonian times aquatic plants invaded the land and fish with lungs and bony fins developed into amphibians. The irony is that the Devonian has often been called the Age of Fish yet at that time 90% of Britain was occupied by the arid ORS continent.
John Downes

Geology / Geology Group Diary (29)
March 14, 2018, 08:11:43 pm
The geology group met at 10.30 am on Wednesday 14 March 2018 at merlin's Bridge Community hall. 18 members present. This month's topic was BRITISH SILURIAN GEOLOGY.

During the Silurian Period (439-409 Ma) Avalonia and Baltica moved towards Laurentia as the Iapetus Ocean closed. Then by the end of Silurian times continental collision occurred as the ocean crust was finally subducted below the Iapetus suture. Scotland became joined to southern Britain which had moved from around 60°S to 20°S since the early Ordovician. A movement of 4500 kilometres over 100 Ma represents a rate of about 4.5 cm per year. However, the most important consequence of the continental collision was to initiate the Caledonian Orogeny. This mountain building movement resulted in the upheaval of a zone of fold mountains stretching from New England through the Scottish Highlands to Greenland and Scandinavia.

The base of the Silurian was defined by Lapworth in the Southern Uplands at Dob's Linn where the graptolitic shales of the Upper  Ordovician (persculptus zone) pass conformably into the of the basal Silurian shales (acuminatus zone). Why is an unconformity not a good place to draw the boundary between two systems? Answer: erosion has removed strata from beneath the unconformity. The top of the Silurian can be recognized most clearly in the Czech Republic around Pridoli where there is a conformable sequence of marine graptolitic shales extending up into the Devonian which begins at the base of the Monograptus uniformis zone. In Britain marine conditions in the Silurian were terminated by the development of the continental deposits known as the Old Red Sandstone. This change to terrestrial conditions took place in the mid Silurian (430 Ma) in Scotland but occurred later in Wales and SW England.  At Red Cliff near Marloes Sands the transition from marine Gray Sandstone Group to continental Red Cliff Formation took place around 424 Ma (beginning of the Ludlow Epoch). The base of the ORS is therefore diachronous, meaning that it was deposited at different times in different areas.

The Welsh Basin that was well established during the Ordovician, continued to exist up to late Silurian times. By studying the fossil content of the sediments laid down it can be shown that the sea advanced eastwards over the Midland Platform to the edge of the Malverns. Shallow water brachiopods such as Lingula and Eocoelia are found in the shelf sediments whereas in the west around Welshpool water depths of up to 100 m are indicated by the presence of Stricklandia and Clorina. Further west around Aberystwyth (Rheidol Gorge) graptolites are present in mudstones suggesting deep water basin conditions.
Pembrokeshire was located on the southern margins of the Welsh Basin, and here the Skomer Volcanic Group represents the products of volcanism that occurred in shallow shelf seas during the early Silurian. Some of the best exposures of lava flows can be seen at Martin's Haven, Marloes Sands and Skomer Island. The lavas are often interbedded with sandstones and calcareous mudstones that contain a variety of brachiopods and corals. This classic sequence is well-exposed at Marloes Sands where in the 1960s pioneering research was carried out on brachiopod communities to demonstrate how they could be used as marine depth indicators.
In Shropshire the Church Stretton Fault follows the edge of the Midland Platform formed of basement Precambrian rocks across which the shallow shelf sea encroached in mid Silurian times (Wenlock & Ludlow epochs). Here a shelly facies of thinly bedded shales and limestones was deposited and these can be seen around Wenlock Edge. This is an area of classic scarp and vale topography where resistant limestone alternates with softer shale. You will see on the cross section that the Wenlock Limestone forms a scarp overlooking Ape Dale (Wenlock Shale) and the Aymestry Limestone forms a scarp overlooking Hope Dale (Lower Ludlow Shale). The limestone is full of coral reefs that were formed in warm shallow seas around 25°S. The bedded limestone wraps round the reefs which are mainly built of corals, sponges and bryozoans, but there are also numerous brachiopods, trilobites and crinoids in the limestone. Some of the well known Silurian fossils include the trilobites Calymene (Dudley bug) and Dalmanites, the colonial chain coral Halysites and the large brachiopod Conchidium.

Silurian strata extends across the Southern Uplands in a belt trending ENE-WSW from the Berwickshire coast (Siccar Point) to Galloway; roughly parallel to the Ordovician outcrop on its northern flank. This is the area made famous by Charles Lapworth (1878) when he used graptolites to demonstrate the structure of the Southern Uplands. He showed that graptolite zone fossils could be used to divide up the shales and greywackes into relatively small units and then deduce that the rocks must be isoclinally folded,  proving that a single condensed graptolitic band was repeated by intense folding  (and reverse faulting). Previously it had been thought that there were numerous shale bands all dipping NW, suggesting that the shale/greywacke sequence was hundreds of metres thick. The Ettrick Valley Fault is a major reverse fault within the accretionary prism of the southern uplands. Remember that these faults relate to the subducting of the oceanic plate beneath the Solway Firth as the Iapetus Ocean finally closed.
Geology / Geology group Diary (28)
February 14, 2018, 07:31:50 pm
The Geology Group met at 10.30 am on 14 February 2018 at Merlin's bridge Village Hall. 19 members present. The topic this month was BRITISH ORDOVICIAN GEOLOGY.
The Ordovician Period (510-439 Ma) was defined by Prof.Charles Lapworth  to resolve a dispute between Adam  Sedgwick (Cambridge Univ.) and Roderick Murchison (Geological Survey). During the 1830s Sedgwick was surveying Cambrian outcrops in North Wales, whilst Murchison was working on the Silurian in the Welsh Borders. The boundary between the two groups of rocks was ill defined and at the time produced great controversy in scientific circles. Finally, in 1879 Lapworth was able to show that there was a distinct faunal content in the strata between the Cambrian and Silurian sequences, hence he defined his new Ordovician system on reliable palaeontological grounds.
During Ordovician times the Iapetus Ocean was closing as the ocean crust was subducted beneath Laurentia (N.America+Scotland) and Avalonia (England&Wales) At the same time the Rheic Ocean between Avalonia and Gondwana was opening. Above the subduction zones island arc vulcanicity was taking place erupting vast quantities of lava and ash in the Lake District and North Wales.
North Wales The Snowdon mountain range is composed largely of Ordovician mudstones and volcanic rocks. In early Ordovician  times (Tremadoc age) marine conditions  existed and great thicknesses of muds were deposited in a geosynclinal basin. These muds were later converted by regional metamorphism into the Tremadoc slates. It is interesting to note that many of the trilobites found in these slates have been distorted by pressure during metamorphism. Island arc volcanoes erupted throughout Arenig and Llanvirn times including Rhobell Fawr and Cadair Idris. The most intensive period of volcanic activity produced some highly explosive eruptions that ejected masses of felsic pyroclastic materials and resulted in the creation of a huge caldera of subsidence in Central Snowdonia. The pyroclastics are of two main types: Airfall materials represent the products of nuée ardente eruptions where a glowing volcanic cloud deposited fine ash into the surrounding sea where it was reworked as a sediment known as air fall tuff. By contrast, ash flow materials roll down the volcanic slopes as incandescent avalanches of gas, steam and lava. They became welded together to form ash flow tuffs or ignimbrites. These often contain glass shards and flattened pumice fragments known as fiamme.
South West Wales. The submarine pillow lavas of Strumble Head provide evidence of island arc vulcanicity on the margins of the Welsh Basin where marine sedimentation produced hundreds of metres of mudstone and volcanic ash bands. The mudstones contain the remains of marine organisms including the well-known 'tuning fork' graptolite Didymograptus murchisoni that is found in the Caerhys shales (Llanvirn stage) at Abereiddi. Inland on the Mynydd Preseli, the Ordovician shales are cut by numerous dolerite intrusions that create rugged tors from which the bluestones of Stonehenge are derived.
Shropshire. The Shelve inlier of Ordovician rocks lies immediately west of the Longmynd and the Pontesford- Linley Fault. The oldest formation is the Stiperstones Quartzite that dips steeply westwards and forms a spectacular ridge of frost shattered tors developed under periglacial conditions at the end of Pleistocene times. The quartzite is a shallow water deposit that is an exceptionally pure white sandstone with a conglomeritic base. The succeeding Mytton Flags are over 900 metres thick; they are hard, blue-grey flaggy siltstones that display conspicuous rectilinear jointing. These joints formed as the sediments dried out. Geologists consider that during late Devonian times a deep seated granite pluton was the source of hydrothermal fluids that were injected into the joints of the Mytton Flags precipitating minerals such as galena (PbS), sphalerite (ZnS) and barytes (BaSO4). Snailbeach mine was a major producer of lead ore from1845 to 1913.
Lake District. The oldest strata in the central Lake District belong to the Skiddaw Group of Lower Ordovician age. The Skiddaw slates, are a sequence of turbidites that have been deformed, folded and cleaved during the Caledonian orogeny.  The graptolitic fauna of these rocks also clearly indicates the deep water marine conditions which existed at this time. However, as the Iapetus Ocean began to close, subduction was taking place below the Solway trench and magmas were generated in the subducted oceanic crust. These magmas fed the granitic intrusions which in turn produced the Eycott and Borrowdale volcanoes that built up some 4000 metres of andesitic lavas, tuffs and agglomerates belonging to the Borrowdale Volcanic Group of Caradoc age. Such volcanic rocks are resistant to erosion and form the highest and most rugged mountain scenery in the Lake District.

South West Scotland is geologically important since it lies on the northern side of the subduction zone below what is now the Solway Firth. As the Iapetus ocean crust was forced down beneath the Laurentia (Scotland) an accretionary prism developed along the leading edge of the continent. This is a sequence of major rock slices cut by reverse faults that can be seen in the steeply dipping Ordovician and Silurian graptolitic shales and greywackes of the Southern Uplands. The strata are also isoclinally folded with the dip consistently to the NW so that it was only through careful mapping that Lapworth first identified the fold structure using graptolites to show that a single shale band is repeated many times by folding. The gorge of Dob's Linn near Moffat is the classic locality where Lapworth worked to unravel the structural geology of the region. At Ballantrae on the coast of Ayrshire, a fragment of  oceanic crust (ophiolite complex) appears above the subduction zone. It is a melange of igneous rocks including gabbro, agglomerate, basaltic pillow lava and serpentinite; typical constituents of dense mafic ocean crust which was pushed up during late Ordovician earth movements.

Geology / Geology Group Diary (27)
January 10, 2018, 08:18:26 pm
The Geology Group met at 10.30 am on Wednesday 10 January 2018 at Merlin's bridge Village hall. The topic this month was BRITISH CAMBRIAN GEOLOGY

During Cambrian times (544-510 Ma) England and Wales formed part of the area known as Avalonia around 60°S on the margins of the continent of Gondwana. The Iapetus Ocean was widening throughout the Cambrian separating Avalonia from Laurentia where Scotland was located around 25°S. The oceanic plates were diverging as basaltic lavas were being extruded from the mid ocean ridge. It is estimated that the ocean was around 5000 kms wide. Most Cambrian rocks in Britain are derived from sediments deposited on the margins of the Iapetus Ocean.
The Welsh Basin existed on the eastern margins of the Iapetus Ocean hence marine conditions existed over much of present day Wales. Turbidite mudstones were deposited on the continental slopes of the subsiding Welsh Basin, as seen in the Harlech Dome, whilst in North Pembrokeshire, in the shallow shelf seas on the margins of the basin, conglomerates and flaggy sandstones were laid down such as the Lingula Flags at Solva and the purple Caerbwdy Sandstone used in the building of St David's Cathedral. Also mudstones rich in the fossilised remains of trilobites and brachiopods give an indication of the abundance of marine invertebrate life that developed during the Cambrian. The well-known trilobite Paradoxides davidis has been found in the Middle Cambrian shales at Porth y Rhaw near Nine Wells. The best specimens of this large trilobite are displayed in the National Museum of Wales.
Rocks of Cambrian age in North Wales are represented by a thick sequence of marine sandstones and shales (most of which are now metamorphosed into slates). The Llanberis Slate outcrops on the NW side of Snowdon where the formation is over 1000 metres thick. The slates were formed as a result of pressure exerted on the Cambrian mudstones during the Caledonian earth building movements at the end of Silurian times. The platy minerals like mica and chlorite in the mudstones were re orientated by pressure that created cleavage planes often perpendicular to the original bedding planes. The slates cleave into thin layers that make excellent roofing material. In the late 19th C at the peak of production, the slate quarrying industry exported Welsh slate across the world. A more modern use of the disused Dinorwic quarry is the pumped storage hydro electric scheme that was opened in 1984. The turbines are hidden in the old slate caverns and they are powered by water released from a high level reservoir which is refilled at night by pumping water up from the lower lake using cheap off peak electricity.
The Harlech Dome is essentially an inlier of Cambrian rocks uplifted as a result of the Caledonian orogeny. The oldest rocks in the centre of the eroded dome are surrounded by a rim of the Rhinog Grits which are topographically higher (Rhinog Fawr 720m). One of the best walks in the Rhinog National Nature Reserve starts at Roman Steps near Cwm Bychan. Manganese and iron pyrites occur in the shales above the Rhinog Grits and gold was mined in the Lower Cambrian rocks between Barmouth and Dolgellau during the 19th C. Note that the oldest rocks are in the centre of the dome and the youngest rocks (that are stratigraphically above the older rocks) outcrop on the periphery of the dome. The Welsh Basin existed on the eastern margins of the Iapetus Ocean hence marine conditions existed over much of present day Wales. Turbidite mudstones were deposited on the continental slopes of the subsiding Welsh Basin, as seen in the Harlech Dome, whilst in North Pembrokeshire, in the shallow shelf seas on the margins of the basin, conglomerates and flaggy sandstones were laid down such as the Lingula Flags at Solva and the purple Caerbwdy Sandstone used in the building of St David's Cathedral. Also mudstones rich in the fossilised remains of trilobites and brachiopods give an indication of the abundance of marine invertebrate life that developed during the Cambrian. The well-known trilobite Paradoxides davidis has been found in the Middle Cambrian shales at Porth y Rhaw near Nine Wells. The best specimens of this large trilobite are displayed in the National Museum of Wales.
An inlier of Cambrian rocks occurs along the line of the Church Stretton fault in Shropshire. Here the basal Cambrian sandstone is known as the Wrekin Quartzite, a pure white sandstone that has a conglomerate at its base.  This contains beach pebbles derived from the local Precambrian Uriconian volcanic hills which must have stood as islands in the Cambrian seas. Ripple bedding within the quartzite is indicative of shallow marine conditions on the margins of the Welsh Basin. The Cambrian sequence in Shropshire is completed with a greenish sandstone rich in glauconite and some thin limestones which have yielded some of the earliest trilobite fossils in Britain. Callavia, found in the Comley Quarry near Church Stretton, belongs to a separate faunal province from those trilobites of similar age found in NW Scotland which lived on the opposite side of the Iapetus Ocean.
In the North West Highlands of Scotland the Cambrian sea laid down the white Eriboll Quartzite unconformably on the eroded Torridonian Sandstone surface. The quartzite can be seen in the road cutting at Skiag Bridge where there is also the distinctive 'pipe rock' Here the beds are full of vertical tubes made by burrowing worms (Skolithus), one of the earliest forms of life in the Cambrian. The trilobite Olenellus lapworthi occurs in the sands and siltstones above the Pipe Rock and this fossil has affinities with North American trilobites because of Scotland's position in Laurentia during Cambrian times. The overlying Durness Limestone is a shallow water deposit containing beds of dolomite (CaMg (CO3)2. The formation of dolomite is due to the later alteration of limestone by magnesium rich groundwater.  The limestone is best seen around the cave of Smoo in the cliffs below Durness village.
The theory of the Cambrian explosion postulates that some 545 Ma ago an explosion of diversity led to the appearance of  large numbers of multi celled organisms within a relatively short period of time (5-10 million years). Most of the major animal groups alive today derive from the Cambrian explosion. Natural selection favoured larger size and the need for hard skeletons to provide structural support; hence the trilobites developed an exoskeleton and shelled creatures such as brachiopods developed. But was the Cambrian explosion as sudden and spontaneous as it appears in the fossil record? Although very few fossil remains have been found in the Precambrian, recent research suggests that organisms were evolving continuously at the molecular level during the Precambrian.
The first indication of a possible explosion of life in the Cambrian came in 1909 when Charles Doolittle Walcott (the original Dr Doolittle) discovered the Burgess Shale in the Rocky Mountains of British Columbia. He unearthed a wide variety of arthropods (trilobites, crustaceans, etc) which were perfectly preserved including soft body parts. The organisms had apparently suffered catastrophic burial being overwhelmed by mudslides at the base of a massive escarpment and buried instantaneously as they sank into the adjacent deep sea basin. The fossils occur as black carbon films on shale and are thought to have been preserved in the stagnant muds where the anoxic environment prevented decay through lack of oxygen. Walcott was head of the Smithsonian Institute in Washington where thousands of his specimens are housed. He was a traditionalist who believed that all the Burgess shale fossils could be assigned to modern day phyla. Since he was a highly respected palaeontologist no one challenged his views at the time. However, in recent decades Walcott's specimens have been re-examined and re-interpreted by researchers and many of the organisms such as the 5 eyed Opabinia and the strange Hallucigenia for example, simply do not belong to any known group of living creatures. In the 1970s Stephen Jay Gould published his book 'Wonderful Life' in which he put forward the theory of punctuated equilibrium which views evolution as long periods of statis punctuated by short bursts of rapid evolution as in the Cambrian explosion. The story of the last 500 years is one of restriction and extinction followed by periods of intense proliferation of a few favoured groups NOT general expansion in range and complexity as implied by the cone of increasing diversity (the tree of life).
Geology / Geology Group Diary (26)
December 13, 2017, 06:58:47 pm
The Geology Group met at 10.30 am on Wednesday 13 December 2017 at Merlin's Bridge Village Hall. This month's topic was 'BRITISH PRECAMBRIAN GEOLOGY''.
Drifting Continents. During the late Precambrian, approximately 700-550 million years ago, the northern regions of the British Isles (Scotland and Northern Ireland) together with Greenland and North America were located on the passive margin of the continent of Laurentia. The southern part of Britain lay on the margins of the sub continent of Avalonia, which was initially part of the super continent of Gondwana.  Avalonia was separated from Laurentia by the Iapetus Ocean that was actively opening and was probably several thousand kilometres wide. However, by the end of the Cambrian Period (510 Ma) the ocean began to close and by early Ordovician times the southern margin of the ocean floor began to be subducted below Avalonia, triggering considerable island arc volcanicity. The Iapetus Ocean continued to close until by the end of the Silurian period (418 Ma) the continents of Laurentia and Avalonia (together with Baltica) collided and the two parts of Britain were united along the Iapetus suture line. The result of this collision was the formation of the Caledonian mountains as the ocean sediments were compressed, uplifted and folded. From the late Precambrian to the end of the Silurian Period, Avalonia had moved from around 60º S of the equator to 20º S; a testimony to the power of plate movements.
NW HIGHLANDS.  This region of Scotland has some of the most spectacular geology and scenery in the whole of the British Isles. It also contains some of Britain's oldest rocks; the Lewisian gneisses that have been radiometrically dated to around 3300 Ma. This compares with 4000 Ma for the oldest Precambrian rocks in the Canadian Shield. Two geological surveyors, Ben Peach and John Horne, conducted detailed mapping in the NW Highlands in the late 19th century that first revealed the large scale thrust structure of the rocks that were emplaced on the Precambrian foreland.
Lewisian Gneiss. The highly deformed and metamorphosed rocks are exposed along the coast from the SE corner of Skye to Cape Wrath and in the Outer Hebridean islands of Lewis and Harris. It is considered that these rocks were originally a sequence of magmatic intrusions and that sometime after 3000 Ma they were subjected to intense heat and pressure and converted into coarsely banded gneiss rich in quartz, feldspar and mica. Later around 2200 Ma the rocks were intruded by a series of mafic dykes that are particularly well exposed near the village of Scourie. About 2 kms north of Laxford Bridge on the A838 there are road cuttings where the Lewisian gneiss is cut by sheets of pink pegmatite that pinch and swell indicating that the gneiss was hot and semi molten at the time of the intrusion. Torridonian Sandstone. The gneisses were uplifted and folded over millions of years and by 1000 Ma a deeply eroded Lewisian landscape had emerged. Then in the late Precambrian the Torridonian sandstone was deposited, resting with a marked unconformity on the eroded Lewisian rocks. The coarse grained red aeolian sandstones are horizontally bedded and have gradually been stripped off  much of the Lewisian basement leaving an exhumed topography with isolated Torridonian mountains such as  Canisp, Suilven, Quinag and Stac Pollaidh. The ice scoured surface of the Lewisian gneiss in coastal Sutherland produces a 'knock and lochan' topography (Cnoc is gaelic for small hill) where numerous small lakes occupy glacial hollows that are interspersed with ice moulded hillocks and erratics of Torridonian sandstone.
Cambrian Sediments. Around 544 Ma the region was inundated by the Cambrian sea which laid down the white Eriboll Quartzite unconformably on the eroded Torridonian sandstone surface. The quartzite can be seen in the road cutting at Skiag Bridge where there is also the distinctive 'pipe rock' Here the beds are full of vertical tubes made by burrowing worms (Skolithus), one of the earliest forms of life in the Cambrian. The Durness Limestone overlies the quartzites. It is interesting to note that the trilobite Olenellus lapworthi occurs in the sands and siltstones above the Pipe Rock and this fossil has affinities with North American trilobites. The plate tectonic evidence suggests that NW Scotland was part of Laurentia (NE North America) in Cambrian times and was separated from the rest of Britain by the Iapetus Ocean.
The Moine Thrust Zone. The coastal region of NW Scotland is separated geologically from the rest of the Highlands by a zone of low angle faults known as the Moine Thrust Zone which was formed during the Caledonian orogeny in late Ordovician times. However, in the 19th C geologists were not aware of this structure and so the rock sequence caused major problems because it appeared that the unaltered Cambrian sediments passed conformably upwards into high grade schists.  The Law of Superposition indicates that the schists must be the youngest rocks in the sequence, but it was hard to explain how unaltered Cambrian sandstones and limestones could be overlain by highly metamorphosed schists. There was much controversy as to how this sequence could have formed. In 1883 Archibald Geikie, Director of the Geological Survey sent two experienced surveyors (on horseback) to this wild remote area of Scotland to carefully map the rock outcrops. As a result Peach was able to demonstrate that the eastern Moine schists (late Precambrian in age) had been thrust westwards over the rigid foreland block of Lewisian gneiss, Torridonian sandstone and Cambrian sediments by a series of large scale low angle thrust faults.(Moine, Glencoul and Sole thrusts). The thrust planes are separated by smaller imbricate faults and duplex structures. Thus we see how the older schists were moved many miles by major thrusts and placed on top of younger Cambrian rocks! A visit to the nature reserve at Knockan Cliff shows the position of the Moine Thrust which is marked there by a fine grained flinty rock called mylonite, produced by rock grinding along the thrust plane.
CHARNWOOD FOREST.  The craggy hills of Charnwood to the north west of Leicester are formed of a late Precambrian inlier composed of volcanics, conglomerates, sandstones and slates. In the 1950s a schoolboy discovered one of the earliest known fossils in layers of volcanic ash dated c.560 Ma. Charnia masoni is a sea pen almost identical to those found in the Edicarna fauna of Australia.
MALVERNS. These hills form a N-S ridge of resistant metamorphosed igneous rocks that separate the Triassic lowlands of Worcester to the east from the Vale of Severn to the west. Gullet Quarry exposes the highly deformed diorites which are in turn cut by numerous dykes and mineral veins. There is an unconformable junction between the late Precambrian Malvernian metamorphic igneous complex and the overlying Silurian sediments
SHROPSHIRE. The oldest rocks in Shropshire are of late Precambrian age. The Uriconian volcanics (566 Ma) form a line of hills extending from the Wrekin south westwards to Caer Caradoc.  These volcanic andesitic lavas were formed in an island arc complex (similar to the Caribbean islands today) above a subduction zone where oceanic crust was forced down at a plate margin. Also rhyolitic ash flows were produced by explosive eruptions from volcanoes lying along the line of the present Church Stretton Fault. Adjacent to the Uriconian volcanics are the Precambrian Longmyndian sediments comprising up to 8000 metres of sandstones and shales forming a deep syncline with an inverted western limb. The higher parts of the sequence contain clasts of Uriconian lavas so these sediments must have been deposited in the shallow seas surrounding the volcanic islands. There is a major unconformity between the eastern (older) and western (younger) Longmyndian sediments followed by a period of intense folding and faulting during the late Precambrian. A good locality on the eastern side of the Longmynd is the Carding Mill Valley near Church Stretton where you can examine the steeply dipping mudstones and sandstones that contain various sedimentary structures including ripple bedding, graded bedding and also bands of volcanic ash. These were deposited under shallow marine conditions where volcanic ash would be erupted from time to time during the later phases of Uriconian vulcanicity. Some of the shales contain fossilised rain spots which were probably preserved as the mud flats dried out along the sea shore.
The Ercall is a small hill on the north east side of the Wrekin. It is significant in that the unconformable junction between the Precambrian igneous rocks and the basal Cambrian sediments is well exposed in the old quarries.  About 560 Ma the Uriconian lavas of the Ercall were intruded by a large mass of granophyre; this is a type of pink granite formed mainly of quartz and orthoclase feldspar. However, banked up against the granophyre are the basal Cambrian sediments known as the Wrekin Quartzite. Since the Cambrian period began around 545 Ma the unconformity represents a time gap of at least some 15 million years. Quartzite is the term used for a metamorphosed sandstone but here the Wrekin quartzite is simply a quartz rich sandstone with a conglomerate at its base. It shows extensive ripple bedding indicative of shallow marine conditions and many of the pebbles in the basal conglomerate are of locally derived rhyolite, tuff and granophyre. The Cambrian shoreline must have surrounded the Precambrian volcanic island arc
ANGLESEY.  The oldest rocks in North West Wales are found in Anglesey and the Lleyn peninsula. These rocks of late Precambrian age are known as the Monian Group (at least 615 Ma) and they are well displayed at South Stack and Rhoscolyn on Holy Island off the west coast of Anglesey. The Monian Group contains turbidite sandstones, volcanic ash and pillow lavas and a remarkable unit called the Gwna melange that is made up of a chaotic mixture of blocks and boulders some of which are several hundred metres across. This represents an enormous submarine slide down the continental slope known as an olistostrome. Pillow lavas are well exposed at Newborough on the south east side of Anglesey along the Menai Strait. Lava flows of this type represent submarine extrusions of basaltic lava that formed large globules or 'pillows' as they react with sea water. Much of the Monian sequence has been strongly metamorphosed into schist and gneiss and intensely folded during orogenesis at the end of Precambrian times when a subduction zone existed in NW Wales.
ST.DAVID'S PENINSULA Using radiometric dating methods the oldest rocks in Pembrokeshire have been dated approximately 643 Ma in the late Precambrian; these occur as a faulted slice of an igneous intrusion that extends E-W through Johnston to the southern side of St. Bride's Bay. In the St. David's peninsula, the volcanic sequence is somewhat younger, around 610-575 Ma and this is intruded by the St. David's granophyre. Roch Castle, Plumstead Rocks and Maiden Castle are outcrops of rhyolite lava that has long been considered to be of late Precambrian age, but now it is assigned by the BGS to the Lower Ordovician.

Geology / Geology Group Diary (25)
November 08, 2017, 08:15:51 pm
The Geology Group met at 10.30 am on Wednesday 8 November 2017 at Merlin's Bridge Village Hall. The topic for the meeting was
Sediments can be deposited in rivers and on the sea floor; they can also be laid down by melting ice or by the wind in arid regions. Lithification is the process by which unconsolidated sediment is converted into a sedimentary rock. The sediment is first compacted by the weight of overlying sediment and water is squeezed out of the pores between the grains. Silica or calcite may be precipitated from solution to form a cement around the grains. Iron oxides also form cement and give the characteristic red colour to sandstones.
There are two main groups of sedimentary rocks (i) Siliciclastic Rocks that are formed mostly of quartz grains or lithic fragments (clasts) and (ii) Carbonate Rocks that are formed by the  precipitation of carbonate minerals in shallow seas.

Sedimentary Processes in the formation of SILICICLASTIC ROCKS

1.Fast flowing mountain streams erode narrow V shaped valleys. Erosion of the channel occurs by abrasion and the fast flowing river transports  boulders, cobbles and pebbles as bedload particularly during floods. Finer sediment is carried in suspension downstream and deposited when current velocity is minimal in lower reaches of river.
2.A lowland river meanders over a flood plain where deposition of silt occurs when water overflows the levees. Lag gravels and coarser sands accumulate on inside of meander bends (point bar) where current is slower.
3.A delta may develop on a coast where a slow moving river is heavily charged with sediment and forced to deposit its load as current velocity drops. Fine silt may be transported many miles out to sea. When sea level fluctuations occur on a deltaic coast repeated fining up sequences or cyclothems of siltstone, shale, sandstone and coal seams are produced
4.In contrast to a meandering river, a braided river will form where the current velocity is high and there is an abundant supply of coarse material forming the bedload. A braided river consists of many channels separated by islands of poorly sorted pebbles and cobbles. This often occurs where a river, fed by glacial meltwater, becomes braided as it emerges from the mountain tract on to the lowland.
5.Sand and mud accumulate along the coast in sheltered bays or behind spits and in lagoons under the influence of long shore drift. Ripple bedding is produced either by wave action in shallow water (symmetrical ripples) or current action (asymmetrical ripples). Cross stratification (cross bedding) occurs as a result of the down current migration of sediment both in rivers or under shallow marine conditions..
6.Turbidity currents occur where sediment is washed out over the continental shelves and descends rapidly down the continental slope through submarine canyons. Poorly sorted, slumped, coarse grained sediment will form greywacke whilst finer muds and sands will accumulate as a fining up sequence (Bouma units).
7.Under aeolian conditions (strong winds) dune bedding (large scale cross stratification) occurs when barchans dunes migrate across a desert surface.
8.Glaciers produce U shaped valleys and debris is transported by ice as it moves downstream. Glacial till (boulder clay) is deposited under an ice sheet as the ice begins to melt.
Shallow shelf carbonate sediments are accumulating today in the Arabian Gulf and off the Bahamas but they were more common in past geological periods.
Common carbonate rocks are:
i).Bioclastic limestones
These are calcareous rocks formed largely of the remains of marine animals such as bivalves, brachiopods and corals. The clasts are often cemented together by calcite. Lower Carboniferous bioclastic limestones were deposited as calcareous sediments on off shore ramps in shallow waters.
ii) Oolitic limestones
These are formed of tiny concentrically layered carbonate grains called ooliths. The calcium carbonate is precipitated in shallow shelf seas under tropical conditions. Today ooliths are being deposited in the Persian Gulf and off the coast of Florida, but in the past oolitic limestones were formed during the Jurassic (eg.Cotswold limestone and Portland limestone)
iii) Chalk
This is a very pure form of limestone made from microscopic skeletal plates (coccoliths) which accumulated as a carbonate rich mud on the sea bed in Upper Cretaceous times. eg the chalk of Beachy Head, Chiltern hills,etc. The Upper Chalk is characterised by the presence of flint nodules which may have been precipitated from silica rich ground waters percolating through the chalk. However, recent research suggests that the flint was formed by the sub surface breakdown of siliceous organisms such as sponges, radiolaria and diatoms during the deposition of the chalk.
iv) Magnesian Limestone (Dolomitic limestone)
This limestone is formed of the mineral dolomite or calcium magnesium carbonate. Most dolomitic limestones are formed by the replacement of the calcium ions in calcareous sediments by magnesium ions when water is evaporated. During Permian times dolomite was deposited as an evaporate mineral in the Zechstein Sea.
Geology / Geology Group diary (24)
October 12, 2017, 06:32:22 pm
The Geology Group met at 10.30 am on Wednesday 11 October 2017 at merlin's Bridge Community Hall. The topic was METAMORPHIC ROCKS.
Metamorphism is the process by which existing rocks are changed in form due to the influence of heat and/or pressure. There are essentially two main types of metamorphism as follows:

1. Contact or Thermal Metamorphism
This takes place around an igneous intrusion where the heat from the molten magma is dissipated into the surrounding rocks. The zone of baking surrounding the intrusion is known as the metamorphic aureole. The intensity of metamorphism declines away from the intrusion as the temperature drops. New minerals are formed by chemical changes within the aureole and these can be used to identify concentric metamorphic zones. For example, a characteristic index mineral is garnet which may be formed when volcanic rocks are thermally metamorphosed.
Mudstones are often changed to a tough flinty rock called hornfels ('hornstone'). The Borrowdale volcanics alongside the Shap granite have been baked to produce hornfels. Limestone is recrystallised to produce marble which may be white like Carrara marble or green as in Connemara marble; impurities such as serpentine or iron minerals produce the colouring.
2. Regional Metamorphism
This is a large scale process involving continental collision (subduction) resulting in partial melting and the deformation of pre existing rocks. The following common metamorphic rocks are formed as the grade of metamorphism increases ie/ the temperature and pressure increases.
i). Slate is produced when fine grained mudstone is subjected to intense pressure and the platy minerals such as mica are orientated at right angles to the direction of pressure. This creates cleavage planes along which the slate splits. These cleavage planes are often at an angle to the original bedding planes of the mudstone.
ii).Quartzite results from the recrystallisation of  sandstone when it is subjected to heat and pressure.
iii) Schist is formed when coarse grained sediments are subjected to higher temperatures and pressures.  Mica is aligned along undulating foliation planes. Garnets often grow along foliation planes in schists during metamorphism.
iv) Gneiss is a high grade metamorphic rock that is often derived from the partial melting of igneous rocks such as granite. This coarse grained rock is characterised by the segregation of its felsic and mafic minerals into bands.
v) Migmatite. Under extreme conditions of regional metamorphism, deep within an orogenic zone, a gneiss will partially melt and recrystallize to form a coarse grained igneous rock called migmatite. It usually consists of alternate light bands of quartz and orthoclase feldspar. and darker bands rich in amphibole and biotite. These bands often appear in tight ptygmatic folds.

Geology / Geology Group Diary (23)
September 14, 2017, 06:49:33 pm
The Geology Group met at Merlin's Bridge Village Hall at 10'30am on Wednesday 13 September 2017. The topic was 'Igneous Rocks and their formation'.
As an introduction we looked at a collection of pebbles from the storm beach at Newgale. The pebbles were graded according to size starting with the fine sand and gravel on the beach and moving up to pebbles and cobbles on the crest of the storm beach. These were moved during powerful winter storms when spring tides raised the water level above mean high water. A variety of rock types could be identified in the pebbles including porphyrytic rhyolite from Ramsey Island, dark dolerite pebbles, white quartz pebbles and a banded gneiss specimen.
Rocks that originate as molten magma within the earth's crust are referred to as Igneous rocks. When magma starts to cool several kilometres below the surface it slowly begins to crystallise forming coarse grained igneous rocks like granite in which you can see the interlocking crystals with the naked eye. When the magma reaches the surface it cools more rapidly producing volcanic lavas that have a fine grained microcrystalline texture like basalt.
There are several types of volcanic eruptions:
Dome volcanoes are formed of viscous felsic magma with a high silica content such as rhyolite and dacite lavas. The Puys of Auvergne are classic dome volcanoes.
Composite volcanoes (or strato volcanoes) are composed of alternate layers of ash and lava such as andesite & trachyte.   Vesuvius (Italy) and Cotopaxi (Ecuador) are typical composite volcanoes. Minor intrusions such as dykes (vertical intrusions) and sills (horizontal intrusions) are often associated with composite volcanoes.
Shield volcanoes have a low profile and cover a wide area. They are formed of fluid basaltic lava with a low silica content.  Mauna Loa in Hawaii is a good example of a shield volcano.
Volcanic eruptions. Dome volcanoes erupt explosively throwing blocks of solid lava and ash out of the crater. Composite volcanoes often produce a Plinian eruption where hot ash and gas rise in a column to form a dense cloud above the cone. Mt.Redoubt in Alaska produced air fall deposits in 2009; the ash fell to the SW of the volcano due to the influence of the prevailing winds. When the eruption column collapses a pyroclastic surge occurs; this is a fast flowing river of pumice and rock fragments in a matrix of ash that rushes down the  slopes of the volcano. When Vesuvius erupted  in AD 79 the town of Pompeii was engulfed in a pyroclastic surge. On Mt Teide in Tenerife beds of white pumice and dark scoria represent the products of alternate pyroclastic flows. When pyroclastic material cools and solidifies it forms a rock called ignimbrite. Composite volcanoes are so called because they consist of alternate layers of ash and lava produced by alternate phases of explosive eruption and quiet lava effusion.
When shield volcanoes erupt they produce a steady flow of basaltic lava which bubbles in the crater as gases are emitted and incandescent debris creates fire fountains. If the lava flow reaches the sea then it cools rapidly on contact with the water forming a skin (reaction rim) around the still molten lava. The term pillow lavas refers to the numerous rounded masses of lava that accumulate on the sea floor.
Classification of Igneous Rocks
Igneous rocks can be classified according to their mineralogical composition.
Felsic Rocks contain > 66% by weight of SiO2. They include Granite and Rhyolite which contain quartz, orthoclase feldspar and mica.
Intermediate Rocks such as Diorite and Andesite contain plagioclase feldspar and amphibole
Mafic Rocks contain < 52% by weight of SiO2. They include Gabbro and Basalt which contain plagioclase feldspar and ferromagnesian minerals (pyroxene and olivine). Ultra mafic rocks like peridotite are rich in olivine.
Igneous rocks which contain large crystals set in a finer grained ground mass are said to have a porphyritic texture. The large crystals are called phenocrysts and these are often used to describe the rock such as quartz porphyry or rhomb porphyry. 'Porphyry' was a highly prized ornamental stone in Ancient Egypt and Imperial Rome, used for statues, vases, columns, etc. The red/purple colour of porphyry was associated with royalty. Another type of igneous rock that is characterised by large crystals is pegmatite which forms from residual silica rich fluids during the later stages of crystallization and is usually found in thick veins fed directly from the magma chamber.
At 10.30 am a group of 15 members plus 2 dogs assembled at the National Trust car park [SR976938] at Broad Haven, near Bosherston. Our first task was to cross the sandy beach to the Carboniferous Limestone outcrop on the north east side of the bay where the beds are dipping southwards and are planed off about 3 metres above the high water mark. This was the height of sea level when the raised beach was deposited during the Ipswichian Interglacial (128-122,000 BP) The rock platform is covered by the remnants of a raised beach deposit of rounded pebbles and cobbles set in a sandy matrix with patches of cemented shelly deposits in places. However, the raised beach is much dissected and eroded by gullies, but it grades upwards into periglacial solifluxion deposits composed of limestone clasts up to 20 cms across. There are also patches of sand rock up to 2 metres thick that appear to be incorporated into the solifluxion deposit. At the top of the section there are exposures of sandy loam and blown sand that form extensive dunes around Broad Haven. The lack of till (boulder clay) at this locality suggests that the area experienced only periglacial conditions during the late Devension (26-10,000 BP) since it was located to the south of the ice front at that time.
We scrambled up over the much weathered Carboniferous limestone to a vantage point on the cliff top and looked across Broad Haven to the sand dunes that extend around the head of the bay. These ridges of sand are formed where there is a wide foreshore that dries out between the tides. Onshore winds blow the dry sand inland so that the resulting dunes tend to migrate landwards until anchored by vegetation such as marram grass. Stackpole Warren is a zone of wind blown sand over a kilometre wide that extends from Broad Haven across to Barafundle Bay. The sand dunes mask the underlying limestone plateau surface which extends across the Pembroke peninsula and represents the marine planation surface eroded during Pliocene times (5-2 Ma).
Broad Haven forms the drainage outlet from what are known as the Bosherston Lakes (also called the Lily Ponds) that were established by the owners of the Stackpole Estate as an ornamental landscape feature in the mid 18th century. This man made landscape of freshwater lakes with dams and sluice gates, was created from the lower reaches of the Broad Haven valleys. Today the area is managed by the National Trust and the Countryside Council for Wales as a National Nature Reserve. Yet this semi natural environment has been created from a geomorphological feature left over from the late Devensian, around 10,000 years ago. The valleys were flooded as sea level rose during the Flandrian marine transgression following the melting of the ice sheets. Deciduous woodlands  gradually developed around the tidal creeks and salt marsh backed by sand dunes covered much of the Broad Haven estuary.
Our last stop before lunch was on the far side of Saddle Point where we viewed the limestone cliffs towards Stackpole Head. The coastline is deeply cut by several narrow inlets such as Sandy Hole and Raming Hole. We could see how the overlying terra rossa (orange sandy clay) had been washed down into the limestone fissures masking the limestone cliffs.
After lunch we gathered at St Govan's free car park and walked a short distance to view the cliffs extending towards St Govan's Head. Here the limestone is massively bedded and dips gently seawards. This means that when large blocks are fractured from the cliffs they slide downslope and accumulate at sea level. Thus the cliff profile tends to be around 45° whilst further west along the coast where the beds are horizontal or dipping inland, the cliffs are vertical and undercut.
However, our main objective for the afternoon was to visit the deep inlets between St Govan's and Saddle Head [SR958928]. When the sea erodes the limestone along faults that are aligned at right angles to the cliff line it produces steep narrow inlets known as geos. The three major geos are Stennis Ford, Huntsman's Leap and Devil's Barn. The most famous of these is Huntsman's Leap, a 30 metre deep vertical chasm that is only a few metres wide at its entrance; narrow enough to be jumped by a galloping horse! However, this is not a recommended practice since the drop to the sea below is over 40 metres. The next inlet to the west is the Devil's Barn that includes a blow hole and a natural arch; and as these features eventually collapse, the geo will be extended inland. Around the head of this geo we examined a thick deposit of terra rossa exposed in the pathway. This orange coloured sandy clay is derived from the slow chemical weathering (solution action) of the limestone leaving a residual deposit of insoluble iron compounds.
Geology / Geology Group Diary (22)
July 13, 2017, 06:12:05 pm
The Geology Group met on Wednesday 12 July 2017 at 10.30am at Merlin's Bridge Community Hall. The topic was 'THE GEOLOGY OF PEMBROKESHIRE (2)
The conformable junction between the Gray Sandstone Group and the basal beds of the Old Red Sandstone can be seen just south of the sea stacks at Red Cliff, Marloes Sands [SM790068]. This locality is highly significant since the change in lithology marks a major environmental change from marine to terrestrial conditions of deposition. The palaeogeography of south-west Wales changed from being a shelf sea environment to one where alluvial plains were crossed by meandering rivers in an arid climate. The carbonate evaporite deposits known as calcretes, found in the red mudstones on Caldey Island and at Freshwater West, suggest an environment of high evaporation and seasonal rainfall. Thick conglomerates and breccias also indicate a rapid deposition by torrential streams under flash flood conditions. The Ridgeway Conglomerate at Sawdern Point in Angle Bay is a good example of this type of deposit. The occurrence of several bands of airfall tuffs in the early Old Red Sandstone of Pembrokeshire provide important marker horizons that can be traced across the Anglo Welsh basin. The Rook's cave tuff is marked by a deep trench on the headland at Manorbier and the Townsend Tuff is well exposed on Little Castle Head to the west of Milford Haven. It is worth noting that the term, Old Red Sandstone, refers to the overall lithology and is not a geological period. In fact the ORS was laid down in a continental environment that began to be deposited in late Silurian times and continued throughout the Devonian period [418-354 Ma].

By the Lower Carboniferous [354-327 Ma] South Wales was covered by a shallow shelf sea with a tropical climate capable of supporting an abundant and varied fauna including corals, crinoids, and brachiopods. The thick marine limestones of South Pembrokeshire form majestic cliffs and the rocks often show features of karstic weathering such as limestone pavements and swallow holes. Collapsed caverns and fault zones sometimes contain gash breccia deposits formed of orange coloured sands and clays containing limestone clasts. The gash breccias are clearly seen in Flimston Bay and Bullslaughter Bay. The Carboniferous Limestone is well exposed along the coast from St Govan's to the Green Bridge of Wales. Here there are sea stacks (Elegug Rocks), natural arches and deep fault guided inlets known as Geos ( Huntsman's Leap). The Variscan earth movements are responsible for folds and faults within the limestone; for example, the Stackpole syncline, the vertical 'organ pipes' around Lydstep Hole and the Flimston wrench fault.

During the Upper Carboniferous [325-290 Ma] the seas were invaded by vast deltas as rivers advanced southwards from the Wales-Brabant landmass. Thick sandstones interbedded with shales were laid down in repeated cycles of sedimentation. Where swamp conditions prevailed on the delta plains, tropical forests provided the raw material for coal seams. The Pembrokeshire coalfield with its E-W axial trend is a westerly extension of the main south Wales coalfield. At Ragwen Point to the east of Amroth the Carboniferous Limestone is overlain by Marros Beds of Namurian age  consisting of thick cross stratified deltaic sandstones. The upper part of the sequence known as the Telpyn Point sandstone passes upwards into the Lower Coal Measures that are well exposed around Amroth. Excellent sections through deltaic distributary channels can be seen in the cliffs near to the inn at Wiseman's Bridge and further west at Saundersfoot is the famous Ladies' anticline; a Variscan structure within the coal measures. In Waterwynch Bay near Tenby there are some splendid examples of soft sediment deformation within the coal measures. Here fractured sandstone blocks appear to have slumped down a depositional slope into liquefied sediment formed under high water table conditions. On the west side of the coalfield at Broad Haven there are several notable Variscan structures such as the much photographed thrust tip fold at Dens Door, Broad Haven. At Rickets Head multiple cyclothems can be observed illustrating the repeated deposition of  a sequence of mudstones, cross stratified sandstones and coal seams. Each cyclothem represents a succession of sediments laid down as the delta builds out or progrades seawards. The delta plain would have been covered by swamp forests and crossed by numerous distributary channels. Eventually the delta plain would become flooded as sea level rose and the next cycle of sedimentation would begin as the sea deposited mud and sand that buried the vegetation which would slowly become compressed into coal.